IDENTITY OF ALL PEIMITIVE EGGS. 1 37 



best microscope, there appears to be no essential difference 

 between the eggs of Man, of the Ape, of the Dog, etc. 

 This does not mean that they are not really different 

 in these different Mammals. On the contrary, we must 

 assume that such difierences, at least in point of chemical 

 composition, exist universally. Even of human eggs, each 

 differs from the other. In accordance with the law of 

 individual variation, we must assume that "all individual 

 organisms are, from the very beginning of their in- 

 dividual existence, different, though often very similar." 

 (Gen. Morph. vol. ii. p. 202). Bat with our rough and 

 incomplete apparatus we are not in a position actually 

 to perceive these delicate individual differences, which 

 must often be sought only in the molecular structure. Yet 

 in spite of this, the remarkable morphological similarity 

 of human and mammalian eggs, which has the appearance 

 of absolute similarity, remains a strong argument in favour 

 of the common descent of Man and the other Mammals. 

 The similar embryo-form bears witness to the common 

 parent-form. On the other hand, there are striking pecu- 

 liarities by which the ripe mammalian egg may be very 

 easily distinguished from the ripe eggs of Birds and other 

 Vertebrates. (Cf. the end of Chapter XXV.) 



The ripe egg of the Bird is especially different, although 

 as a primitive Qgg (Fig. 10, ^ it was entirely similar to 

 that of Mammals. But the egg-cell of the Bird at a later 

 period, though while still within the oviduct, absorbs a mass 

 of food which it elaborates into the large and well-known 

 yellow yelk. If a very young egg from the ovary of a hen 

 is examined, it is found to be exactly like the young egg- 

 cells of Mammals and other animals (Fig. 10). But it 



