SECT. l] 



PHYSICO-CHEMICAL SYSTEM 



313 



The position of affairs may perhaps be summarised by saying that 

 it is only the birds which have been successful in producing an egg 

 really well stocked with fat, though the reptiles clearly show an 

 approximation to this achievement. Does this mean that the storage 

 of fat in the egg is particularly associated with terrestrial embryos? 

 The facts and arguments to be brought forward in later chapters (see 

 Sections 7-7, 9-15 and 1 1-8) make this hypothesis a very likely one, but, 

 as Table 31 shows, the silkworm (the only representative of terrestrial 



Table 31. Protein/ fat ratio in various eggs. 



VERTEBRATA 



arthropods) does not seem to have succeeded in storing fat in its 

 egg to any great extent. It might, of course, be argued that this was 

 one of the factors which prevented the insects attaining any con- 

 siderable size and rivaling reptiles and mammals for the possession 

 of the land. The mammals gave up the heavy fat storage in the egg 

 when they invented viviparity and the fully developed placenta. In 



