A note on the effect of a commercial drying process on the long chain fatty acids of milk

The role of the essential fatty acids, linoleic and arachidonic acids, in human nutrition has been widely studied, particularly with respect to their possible action in preventing arterial disease in adults, e.g. Kinsell (1963). In addition, Hansen, Haggard, Boelsche, Adam & Wiese (1958) have emphasized the importance of linoleic acid in the nutrition of infants. Although milk fat contains only relatively small amounts of linoleic and arachidonic acids, the part played by milk and milk products in contributing essential fatty acids to the human diet has received considerable attention (Combes, Pratt & Wiese, 1962; Kon, 1962; Hansenet al.1963; Garton, 1964). However, despite the initial dependence of many infants on dried milk as an exogenous source of essential fatty acids the effects of commercial drying processes on the constituents of milk fat have not been much investigated. A comparative study of the fatty acid compositions of raw and dried milk was therefore undertaken.

The effects of dietary tallow and cottonseed oil on milk fat secretion in the cow

The effects of the isocaloric replacement of part of the dietary concentrate mixture by either tallow or cottonseed oil on the yield and composition of the milk fat was investigated in 2 feeding experiments with a total of 26 cows in midlactation. The concentrates were given with high- or low-roughage diets that supplied either 9·1–9·5 or 1·8–3·2kg hay/day.

In expt 1, the addition of cottonseed oil to the high-roughage diet increased the yield of milk fat during the first 8 days but decreased it during the last 4 days of a 28-day feeding period. In contrast, the inclusion of tallow in the high-roughage diet resulted in an increased yield of milk fat that was sustained throughout the period of 28 days. In expt 2, the intake of dietary fat was less than it was in expt 1 and the inclusion of tallow or cottonseed oil in either the high- or low-roughage diets had little effect on the yield of milk fat.

The inclusion of either fat in the diet increased the yields and percentages of stearic and oleic acids and, in general, decreased the yields and percentages of the medium-chain fatty acids (10:0, 12:0 and 14:0) in the milk fat. The addition of tallow to the diet did not appear to alter the yields of the short-chain fatty acids (C4–C8, 4:0, 6:0 and 8:0) in the milk fat. When the low-roughage diets resulted in a decreased yield of milk fat, the secretion of all the fatty acids in the milk fat was reduced, but the reduction in the secretion of oleic acid was less than the reductions in the secretions of the other constituent fatty acids.

Dietary fatty acids and the plasma phospholipids of the lactating cow

Cows were given either a low-fat concentrate mixture or a concentrate mixture that contained 5 or 10% ‘stearic acid’ (85% pure) or 10% ‘palmitic acid’ (85% pure). The concentrate mixtures were given with a high roughage diet that supplied 4·4 kg of hay and 2·7 kg of sugar-beet pulp/day. Blood samples were taken from the cows on each dietary treatment and the plasma phospholipids were analysed.

Phosphatidyl choline accounted for 70% of the plasma phospholipids when the cows were given the low-fat diet and about 86% of the plasma phospholipids when the cows were given the diets supplemented with the fatty acids. The inclusion of the fatty acids in the concentrate mixtures decreased the relative proportions of the plasma phosphatidyl ethanolamine, sphingomyelin and lysophosphatidyl choline but the relative proportion of the plasma phosphatidyl serine remained unchanged.

When the diet was supplemented with stearic acid the concentrations of 18:0 and 16:0 in the phosphatidyl choline were unaltered but the concentration of 18:1 was increased and the concentration of 18:2 was decreased. When the diet was supplemented with palmitic acid the concentrations of 16:0 and 18:1 in the phosphatidyl choline were increased and the concentrations of 18:0 and 18:2 were decreased. In contrast, the inclusion of stearic acid in the diet increased the concentration of 18:0 in the phosphatidyl serine and decreased the concentration of 16:0; the concentrations of 18:1 and 18:2 were unchanged. The fatty acid composition of the plasma phosphatidyl ethanolamine was unaffected by dietary treatment. The effects of diet on the fatty acid compositions of the phosphatidyl choline and lysophosphatidyl choline were similar. Supplementation of the diet with stearic acid increased the concentrations of 18:0 and 18:1 in the plasma sphingomyelin and decreased the concentrations of 16:0 and 18:2. The addition of palmitic acid to the diet increased the concentration of 16:0 in the sphingomyelin but it decreased the concentrations of the other constituent fatty acids.

Observations on the pattern on biohydrogenation of esterified and unesterified linoleic acid in the rumen

1. Studies have been made of the effects of different concentrations of either free or esterified linoleic acid on the biohydrogenation of linoleic acid by rumen micro-organisms in vitro. A comparison has been made with the changes which occurred in the fatty acid compositions of rumen free fatty acids and plasma triglycerides of sheep given intraruminal infusions of linoleic acid or maize oil.

2. In the in vitro experiments, with increasing concentrations of 18:2 added as the free fatty acid, a decreasing proportion of this 18:2 was hydrogenated to 18:0 andtrans-11-octadecenoic acid accumulated. The accumulation of large amounts oftrans-11-octadecenoic acid was accompanied in all instances by the accumulation of a conjugated diene identified ascis-9,trans-11-octadecadienoic acid. There appeared to be a product–precursor relationship between the conjugated diene and thetrans-11 monoene.

3. When linoleic acid was presented in vitro as the triglyceride, the extent to which hydrogenation occurred was, in all instances, greater than when equivalent amounts of 18:2 were presented as the free acid. Only small amounts of thecis-9,trans-11 diene were detected, and there was no apparent product–precursor relationship between this conjugated diene and the C18monoenoic acids. The C18monoenoic acids that accumulated consisted of bothcisandtransisomers; thecisisomers consisted largely ofcis-9- andcis-11-octadecenoic acids, which together comprised about 30% of the C18monoenoic acids present.

4. The infusion of free linoleic acid into the rumen of sheep resulted in an increase in the proportion of total 18:1 and a decrease in the proportions of 16:0 and 18:0 in the total rumen free fatty acids. This increase which occurred in the concentration of 18:1 consisted predominantly of thetrans-11 isomer. A concomitant increase in the concentration of the C18trans-11 acid was observed to occur in the fatty acids of the plasma triglycerides. Infusion of maize oil into the rumen of sheep resulted in little change in the fatty acid compositions of either the free fatty acids in the rumen or the triglycerides of the plasma.

5. The findings in vitro and in vivo are discussed with reference to each other and with reference to the possibility that biohydrogenation of 18:2 derived from the triglyceride proceeds by a different pathway from that of 18:2 presented as the free acid.

Differences in the metabolism of esterified and unesterified linoleic acid by rumen micro-organisms

1. Sheep were given intraruminal infusions of maize oil or linoleic acid and samples of contents were taken from the rumen and abomasum at different times after the infusions. Hydrolysis of the maize oil occurred in the rumen with the production of mono- and di-glycerides as intermediates. Linoleic acid derived from the maize oil was hydrogenated to stearic acid. When linoleic acid was infused into the rumen, little or no stearic acid was produced and octadecenoic acid accumulated.

2. When linoleic acid or maize oil was incubated with rumen contents in an artificial rumen and samples of the reaction mixtures were taken from the apparatus after various time intervals, the results were similar to those obtained in vivo, except that the hydrolysis of maize oil did not give rise to mono- and di-glycerides.

3. These results are discussed in relation to previous findings on the effects of intraruminal infusions of maize oil or linoleic acid on the fatty acid composition of the blood triglycerides of sheep.

Dietary linoleic acid and the plasma phospholipids of the rabbit

1. Groups of rabbits were given diets containing different proportions of butterfat and maize oil. After the animals had been given the experimental diets for 40 weeks the plasma phospho- lipids were fractionated and the fatty acid composition of each fraction was determined.

2. Phosphatidyl choline and lysophosphatidyl choline accounted for about 75 and 12% respectively of the total plasma phospholipids: phosphatidyl ethanolamine, sphingomyelin and phosphatidyl serine accounted for only about 5.3, 5.0 and 2.6% respectively. Changes in the linoleic acid content of the diet had little effect on the relative proportions of the individual plasma phospholipids, but there was an over-all decrease in the concentration of total phospholipids in the plasma as the linoleic acid content of the diet was increased from 0.25 to 10.6%.

3. When the diet contained 0.25 % linoleic acid, the linoleic acid:oleic acid ratio in the phosphatidyl choline (1.3) was similar to that in the phosphatidyl ethanolamine (1.2), but as the linoleic acid content of the diet was increased to 10.6% the linoleic acid:oleic acid ratio in the phosphatidyl choline increased to 48, whereas that in the phosphatidyl ethanolamine increased only to 2.2. Increases in the linoleic acid content of the diet resulted also in increases in the linoleic acid:oleic acid ratios in the phosphatidyl serine, lysophosphatidyl choline and sp hingomyelin.

4. When the linoleic acid content of the diet was increased, the stearic acid:palmitic acid ratio in the phosphatidyl choline increased, whereas it decreased in the phosphatidyl ethanol- amine and remained relatively unaltered in the phosphatidyl serine. The stearic acid contents of the lysophosphatidyl choline and sphingomyelin were unaltered by dietary treatments, but the palmitic acid content of these two phospholipids decreased as the linoleic acid content of the diet increased.

5. The results are discussed in terms of the metabolic relationships that exist between the various phospholipids.

The incorporation of linolenic and linoleic acids into the plasma lipids of sheep given intra-abomasal infusions of linseed oil, maize oil or linoleic acid

1. The fatty acid compositions of the plasma cholesteryl esters, phospholipids, triglycerides and unesterified fatty acids were determined in three sheep at various times after they had been given intra-abomasal infusions of emulsions of linseed oil, maize oil or linoleic acid.

2. The concentrations of linolenic acid or linoleic acid in the plasma triglycerides began to increase 1.5 h after infusion of the emulsions had begun. As the concentration of linolenic or linoleic acids in the plasma triglycerides increased, the concentrations of palmitic and stearic acids decreased, hut there were no appreciable changes in the concentrations of oleic acid.

3. The concentrations of linolenic or linoleic acid in the plasma phospholipids and cholesteryl esters did not begin to increase until 8–9 h and 24–25 h respectively after the infusions of the emulsions had begun.

4. It is suggested that, after absorption from the small intestine of the sheep, linolenic and linoleic acids are transported in triglyceride form to the liver where the triglycerides are partially or completely hydrolysed. These C18polyunsaturated acids are then preferentially utilized for the synthesis of phospholipids and cholesteryl esters but not for the re-synthesis of triglycerides.

The effect of diets containing different proportions of butterfat and maize oil on plasma lipids and aortic atherosis in rabbits

1. Seven groups of male rabbits (thirteen to fifteen per group) were givenad lib. a diet consisting of 80 parts of a low-fat basal ration to which were added 20 parts of fat in the form of butterfat, maize oil or various mixtures of the two.

2. After 40 weeks, samples of blood were taken and the animals were killed. The concentrations of total cholesterol and phospholipid in the plasma and the fatty acid compositions of the plasma cholesteryl esters and phospholipids were determined. The degree of atheromatous degeneration was determined in the aortas.

3. Little aortic atherosis was observed in the rabbits given the diets containing 0, 10 and 12% butterfat, but as the butterfat content of the diet was increased above 12%, pronounced increases occurred in the extent of atheromatous degeneration of the aorta. The concentration of cholesterol in the plasma increased progressively as the butterfat content of the diet was increased and the maize oil content was decreased. There was an inverse curvilinear relationship between the degree of aortic atherosis and the phospholipid: cholesterol ratio in the plasma.

4. The concentration of linoleic acid in the plasma cholesteryl esters was inversely related to the concentration of cholesterol in the plasma, and the concentration of linoleic acid in the plasma phospholipids was inversely related to the concentration of phospholipids in the plasma. There was a positive linear relationship between the stearic: palmitic acid ratio and the linoleic: oleic acid ratio in the plasma phospholipids.

Factors affecting the polyunsaturated fatty acid content of the plasma lipids of sheep

1. In Expt 1, four adult wether sheep were given diets of hay or dried grass in an alternating sequence of feeding treatments. Each treatment period lasted for 20 days and blood samples were taken from the sheep on the last day of each period. When the diet of hay (linoleic: linolenic acid ratio 1.40) was replaced by one of dried grass (linoleic: linolenic acid ratio 0.28), there was an increase in the concentration of linolenic acid and a decrease in the concentration of linoleic acid in the plasma cholesteryl esters and phospholipids. There was an increase in the concentration of stearic acid and a decrease in the concentration of palmitic acid in the plasma triglycerides. Dietary change did not affect the composition of the plasma unesterified fatty acids.

2. In Expt 2, two adult wether sheep, each with a rumen fistula, were given daily intraruminal infusions of 60 g of ‘linolenic’ or ‘linoleic’ acids (both about 70% pure) over a period of 5 days. The infusion treatments were then reversed. Blood samples were taken at the end of each infusion period. Intraruminal infusions of ‘linolenic’ acid increased the concentration of linolenic acid in the plasma cholesteryl esters and phospholipids and increased the concentration of stearic acid in the plasma triglycerides. The infusions of ‘linoleic’ acid increased the concentration of linoleic acid in the plasma cholesteryl esters and phospholipids. No appreciable changes were observed in the composition of the plasma unesterified fatty acids.

Comparison of the lipid compositions of the aortic lesions in rabbits given different atherogenic diets

1. The compositions of the lipids in the atheromatous lesions of rabbits have been studied in an experiment in which two groups of rabbits (twelve/group) were given different atherogenic diets for a period of 38 weeks. One group of rabbits was given a diet in which 35% of the total calories was derived from butterfat and the other group was given a diet in which 48% of the total calories was derived from starch.

2. At the end of the feeding period the rabbits were killed and the atheromatous plaques were dissected out from the aortic intima of each rabbit. The lipid compositions of the aortic lesions were determined and the results compared with the compositions of the plasma lipids.

3. In the lipids of the aortic lesions in the rabbits given the high-butterfat diet, the proportions of cholesterol and cholesterol esters were higher and the proportion of triglycerides was lower than in the lipids of the aortic lesions in the rabbits given the high-starch diet. Irrespective of dietary treatment, free cholesterol constituted a higher proportion of the lesion lipids than it did of the plasma lipids.

4. In the rabbits on either dietary treatment, the lesion cholesterol esters contained higher concentrations of oleic acid and lower concentrations of linoleic acid than did the plasma cholesterol esters. In the lesion phospholipids there were higher concentrations of palmitic, palmitoleic, stearic and oleic acids and lower concentrations of linoleic and arachidonic acids than in the plasma phospholipids.

5. In both groups of rabbits the phospholipids of the aortic lesions contained higher proportions of cephalin and sphingomyelin and lower proportions of lecithin than did the plasma phospholipids.

6. The results of this investigation are discussed in relation to the possible origins of the lipids in the aortic lesions of the experimental rabbits.

The transport of phospholipids from the yolk to the yolk-sac membrane during the development of the chick embryo

A study was made of the concentrations and fatty acid compositions of the various phospholipid fractions in the yolks and yolk-sac membranes at different stages in the development of the chick embryo. Phosphatidyl choline was the major component of both the yolk and membrane phospholipids. The composition of the yolk phospholipids was broadly similar to that of the membrane phospholipids. In both the yolk and membrane phospholipids, the proportions of phosphatidyl choline and phosphatidyl ethanolamine tended to decrease, whereas the proportions of phosphatidyl serine, sphingomyelin, and lysophosphatidyl choline tended to increase as incubation proceeded. The fatty acid composition of the yolk phosphatidyl choline did not change during incubation, but there was a progressive decrease in the palmitic acid: stearic acid ratio in the membrane phosphatidyl choline. The changes in the fatty acid composition of the yolk phosphatidyl ethanolamine indicated that there was a preferential absorption from the yolk of phosphatidyl ethanolamine with stearic acid in the α position and arachidonic or docosahexaenoic acids in the β position. The lysophosphatidyl choline in the yolk and membrane evidently consisted of mixtures of the 1-acyl and 2-acyl isomers. There was no evidence that extensive breakdown and resynthesis of phospholipids occurred during the transport of lipids from the yolk to the yolk-sac membrane.

The partition of lipids between the yolk and yolk-sac membrane during the development of the chick embryo

A study was made of the concentrations and fatty acid compositions of the various lipid fractions in the yolks and yolk-sac membranes at different stages in the development of the chick embryo. Between the 13th and 17th days of incubation, the transport of lipid from yolk to membrane was so active that on day 17 the yolk-sac membrane contained as much lipid as did the yolk. Free fatty acids, monoglycerides, and diglycerides could not be detected in more than trace concentrations in either the yolk or membrane lipids. Comparisons of the fatty acid compositions and the positional distributions of the various fatty acids in the triglycerides of the yolks and yolk-sac membranes suggested that the major proportion of the yolk triglycerides was absorbed intact by the yolk-sac membrane. There were increases in the esterified : free cholesterol ratios in the yolks and yolk-sac membranes as incubation proceeded but in the membranes, the ratios were markedly greater than those in the yolk. The transport of lipid from yolk to membrane evidently involved extensive esterification of the yolk cholesterol, mainly with oleic acid.

The liver phospholipids of the developing chick embryo

The concentrations and fatty acid compositions of the individual phospholipids in the livers of chick embryos on the 13th, 15th, 17th, 19th, and 21st days of incubation were compared with the concentrations and fatty acid compositions of the individual yolk phospholipids. The liver phospholipids contained higher proportions of phosphatidyl ethanolamine, phosphatidyl serine, and diphosphatidyl glycerol, and lower proportions of phosphatidyl choline, than did the yolk phospholipids. There was a constant increment (0.96 mg/day) of phosphatidyl choline in the liver during the period of incubation studied. The phosphatidyl choline, phosphatidyl ethanolamine, and phosphatidyl serine of the liver generally contained higher concentrations of stearic, arachidonic, and docosahexaenoic acids, and lower concentrations of palmitic and oleic acids than did these phospholipid fractions in the yolk. The fatty acid compositions of the sphingomyelin in the liver and yolk were similar. The most pronounced changes in the fatty acid composition of the liver phospholipids during embryonic development were observed in the phosphatidyl choline fraction. These changes suggested that the α-palmitoyl-β-arachidonyl phosphatidyl choline in the liver was gradually replaced by α-stearoyl-β-linoleoyl phosphatidyl choline and a-stearoyl-β-docosahexaenoyl phosphatidyl choline.

The effect of an atherogenic diet on plasma lipid composition and aortic atherosis in two strains of New Zealand White rabbit

1. Susceptibility to dietary induction of hypercholesterolaemia and aortic atherosis was compared in two groups of male New Zealand White rabbits. 2. Twelve rabbits were purchased from one breeding establishment (group I) and twelve from another (group 2). On arrival at the laboratory six animals from each group were killed and the aortas were removed. Blood samples were taken from the remaining twelve animals and then they were givenad lib.for a period of 40 weeks an atherogenic diet containing 20% butterfat. During this period food intake and body-weight were recorded. At the end of the period blood samples were taken from the animals and immediately afterwards they were killed and the aortas were removed. 3. No atheromatous lesions were found in the aortas of the rabbits in either group killed at the beginning of the experiment. There were no differences between the two groups of animals with respect to body-weight or concentration of cholesterol in the plasma at the beginning of the experiment, food intake during the experiment or body-weight at the end of the experiment. At the end of the experiment, the degree of aortic atherosis in the rabbits of group I was considerably greater than that in the rabbits of group 2. 4. At the end of the experiment the concentrations of total lipids, free cholesterol, esterified cholesterol and phospholipids in the plasma of the rabbits in group 1 were significantly higher than the corresponding concentrations of these lipid components in the plasma of the rabbits in group 2. The concentrations of palmitic, stearic and linoleic acids in the cholesterol esters and the concentration of palmitic acid in the unesterified fatty acids in the plasma of the rabbits in group 2 were significantly higher than the corresponding concentrations of these fatty acids in the plasma cholesterol esters and unesterified fatty acids in the rabbits of group 1. 5. It is concluded that these differences in response to the atherogenic diet were reflections of the differences in the susceptibilities to the dietary induction of hypercholesterolaemia and atherosis of the two differents strain of rabbit that had been established by the two commercial breeders. Such differences in susceptibility could readily explain certain discrepancies in the results of various research workers engaged in this field of investigation.

The effect of diet on the compositions of the triglycerides and unesterified fatty acids isolated from the plasma, liver and adipose tissues of rabbits

1. Groups of male rabbits (ten to thirteen per group survived) were givenad lib. a diet consisting of 80 parts of a low-fat basal diet to which were added: for group I, 20 parts maize oil; for group 2, 20 parts butterfat; for group 3, 10parts maize oil and and 10parts butter- fat; for group 4, 0.47 parts maize oil and 43.1 parts wheat starch; and for group 5, 10.2parts maize oil and 21.6 parts wheat starch. The animals in group 6 were givenan ordinary com- mercial rabbit diet. 2. The rabbits were given the various diets for 38 weeks, after which a sample of blood was taken. The rabbits were then killed and the liver and a sample of perine- phric adipose tissue were removed from each animal. The plasma, liver and adipose tissue lipids werefractionated on columns of Florisil and the fatty acid compositions of the tri- glyceride and unesterified fatty acid fractions were determined by gas-liquid chromatography.

3. The effects of the different diets on the composition of the unesterifiedfatty acids in the plasma were very similar to the effects of the diets on the fatty acid composition of the plasma triglycerides, but in the plasma unesterified fatty acids the concentration of stearic acid was consistently higher and the concentration of linoleic acid was consistently lower than in the plasma triglycerides. 4. There appeared to be a positive rectilinear relationship between the concentration of stearic acid in the plasma triglycerides and the concentration of triglycerides in the plasma. 5. In the triglycerides of the plasma, the concentrations of palmitic and stearic acids were consistently greater and the concentration of linoleic acid was consistently less than the corresponding concentrations of these fatty acids in the triglycerides of the adipose tissues. In the two groups of rabbits given low-fat diets the fatty acid composition of the liver trigly- cerides was almost identical with that of the plasma triglycerides. 6. In each of the six groups of rabbits the composition of the unesterified fatty acids in the plasma was identical with that of the unesterified fatty acids in the adipose tissues. Inthe unesterified fatty acids of the liver the concentrations of linoleic andarachidonic acids were consistently greater and the concentrations of myristic, palmitic and palmitoleic acids were consistently less than the corresponding concentrations of these acids in the unesterified fatty acids of the plasma and adipose tissues. 7. The results of this investigation are discussedin the light of recent con-cepts on the metabolic relationships between the unesterified fatty acids and triglycerides of the liver, plasma and adipose tissues.

Metabolism of the yolk phospholipids by the developing chick embryo

A study was made of the concentrations and fatty acid compositions of the various phospholipids in the yolks of fertile unincubated eggs and in the yolks of eggs that had been incubated for 13,15,17,19, and 21 days. Phosphatidyl choline and phosphatidyl ethanolamine accounted for 69 and 24% respectively of the total phospholipids present in the yolk of the unincubated egg. The remaining 7% was accounted for by small amounts of phosphatidyl serine, sphingomyelin, and a phospholipid fraction that was tentatively identified as diphosphatidyl glycerol. Although the percentage of total phospholipids in the total yolk lipid did not vary during incubation, there was a pronounced increase in the phosphatidyl choline: phosphatidyl ethanolamine ratio as incubation proceeded. The phosphatidyl ethanolamine fraction was the only phospholipid fraction present in the yolk that showed any consistent change in fatty acid composition during incubation. The concentration of docosahexaenoic acid in the fatty acids of the yolk phosphatidyl ethanolamine decreased from 8% on day 0 to 1.6% on day 21. It is suggested that the developing embryo preferentially absorbs from the yolk a phosphatidyl ethanolamine fraction that is relatively rich in docosahexaenoic acid.

STUDIES ON THE LIPID METABOLISM OF THE CHICK EMBRYO

A study was made of the amounts and fatty acid compositions of the cholesterol esters, phospholipids, and triglycerides present in the yolk of the fertile unincubated egg and in the yolk, liver, and extrahepatic tissues of the chick embryo at various stages of development. Esterification of cholesterol, mainly with oleic acid, occurred in the yolk during incubation. There appeared to be a preferential absorption from the yolk sac of phospholipids rich in docosahexaenoic acid. Considerable amounts of cholesterol esters, of which 80% was cholesterol oleate, accumulated in the embryonic liver. The liver phospholipids contained more stearic, arachidonic, and docosahexaenoic acids, and less palmitic and oleic acids, than did the yolk phospholipids. Docosahexaenoic acid occurred in a surprisingly high concentration in the liver triglycerides. The extrahepatic triglycerides contained more palmitic and C18polyunsaturated acids, but less docosahexaenoic acid, than did the liver triglycerides. The concentration of oleic acid in the extrahepatic cholesterol esters was much less than in the liver cholesterol esters. The extrahepatic phospholipids contained more arachidonic and docosahexaenoic acids, but less oleic acid, than did the yolk phospholipids. The implications of these findings are discussed in relation to the general lipid metabolism of the chick embryo.