ii84 FAT METABOLISM [pt. m 



a rise in total fat from 475 to 5-25 per cent, of the wet weight during 

 the 40 days before hatching. And whereas one egg of the Atlantic 

 salmon {Salmo salar) contained 6 mgm. fat at the beginning of de- 

 velopment it contained 7 mgm. at hatching (65 days). 



1 1 -6. Fat Metabolism of Mollusc, Worm, and Echinoderm 

 Eggs 

 The synthesis of fatty acids occurs again in the metabolism of the 

 snail egg [Limnaea stagnalis), and it was here indeed that it was first 

 recognised. For in 1853 F. W. Burdach estimated the fat-content of 

 the eggs of the snail from fertilisation to hatching, and the results were 

 published in his thesis De Commutatione Substantiarum Proteinacearum in 

 Adipem at Konigsberg in that year. His technique was, of course, 

 that in general use at the time, and therefore probably very in- 

 accurate, but the figures were as follows : 



Before After 



development development 



Absolute weight in milligrams per egg Dry weight ... 335"5 2i8-o 



Neutral fat ... 2-25 3-5 



Fat % of dry weight o-66 i-86 



On the other hand, Faure-Fremiet, in his comprehensive study of 

 the Ascaris egg, observed a diminution of the fraction which he called 

 "fat", but which was really a mixture of neutral fats and ascarylic 

 acid. This fraction fell from 26 per cent, of the dry weight of the egg 

 at the beginning of development to 22-5 per cent, at the end, i.e. a 

 loss of 3*5 per cent, of the dry weight. Then the work of Ephrussi & 

 Rapkine on the developing egg of the sea-urchin, Paracentrotus lividus, 

 showed that there was a diminution in the total fatty acids, thus: 



Hours from fertilisation 



12 40 



Total fatty acids o (gastrula) (pluteus) 



% dry weight ... 2i-2 19-55 ^TA 



% wet weight ... 4-81 4-43 3-69 



And Payne has shown that fertilisation does not affect the iodine 

 value of the fatty acids in the egg of^ Arbacia. 



11-7. Fat Metabolism of Insect Eggs 



The silkworm egg definitely shows a negative balance of total fatty 

 acids. Tichomirov in 1885 obtained the following figures: 



