THE KINSHIP OF LIFE. 



39 



a larger amount of 'food yolk ' stored up to supply the 

 wants of the growing embryo till the time comes when 

 it shall be able to shift for itself. Protoplasm is active, 

 ' food yolk ' passive, and the relative amounts of these 

 two and the positions which they occupy in the egg 

 affect, in a purely mechanical manner, the segmentation, 

 and interfere with or destroy its typical regularity. In 

 the egg of the common hen this ' food yolk ' forms 

 almost the whole of the yolk, the really important pro- 

 toplasm occurring only in the lighter yellow spot, which 

 is always uppermost in the egg. Taking it for granted 

 that this amount of food yolk influences the character 

 of the early stages of development (a point easily proved 

 by the embryologists), let us consider a special case in 

 which conclusions drawn from development have re- 

 ceived later confirmation from other sources. 



" In the mammals the eggs are very small and con- 

 sist of pure protoplasm, food yolk being entirely absent. 



Indeed, nourished by the mother, as the 

 The egg of the Qf most Q these formg nQ 



mammal. . 



store of food yolk is necessary. Hence, 



on a priori grounds, one would say that the segmenta- 

 tion of the mammalian egg would be regular in its char- 

 acter. When, however, naturalists came to study the 

 development of the mammalian egg, it was found that 

 in its early stages it presented (in eggs without food 

 yolk) some astonishing peculiarities. How to explain 

 these peculiarities was a problem. If, however, it were 

 assumed that the mammals have descended from forms 

 with larger eggs, and that in the course of evolution 

 they have lost the yolk but had retained the tendencies 

 of development, the explanation were easy. This ex- 

 planation, however, seemed very improbable, for it had 

 been held, on grounds of structure, that the mammals 

 must have descended from the batrachia, a group con- 



