THE PEDIGREE OF BIRDS 349 



§ 3. Resemblances in the Development of Birds and 



Reptiles 



Birds and reptiles have the same type of egg, the true 

 ovum or egg-cell becoming enormously dilated with yolk 

 and surrounded by albumen and a shell. The egg of a 

 crocodile is like the egg of a goose, both externally and 

 internally. On the top of the yolk, in both cases, there Ues 

 a drop of living-matter with a nucleus, and this segments 

 after fertilisation into a disc of cells or blastoderm. In other 

 words, there is in birds and reptiles a process of partial 

 discoidal segmentation. The early embryonic stages are 

 closely alike. In both, for instance, there appears along the 

 dorsal median hne a " primitive streak," and soon afterwards 

 in the same line a primitive groove — the beginning of the 

 establishment of the nervous system. In both, again, there 

 appear at an early date two foetal membranes — the protective 

 amnion and the respiratory, excretory, and nutritive allantois. 

 The unhatched bird and the unhatched reptile breathe in 

 the same way, for the oxygen that finds its way through the 

 shell is absorbed by the blood-vessels of the allantois spread 

 out underneath. In short, the early development of the bird 

 closely resembles the early development of the reptile. For 

 days they travel, as it were, side by side along the same broad 

 highway, and it is only gradually that they diverge along 

 avian and reptilian paths. In the case of the chick it is not 

 till the sixth day that definitely " avian " characters begin to 

 appear. 



It is often from relatively unimportant details that the 

 most convincing evidence comes. Thus in the embryo of the 

 very old-fashioned South American Hoatzin (Opisthocomus) 

 there is a paw-like hand with three clawed fingers, and there 

 are numerous elements in the wrist. It was this sort of 

 evidence that led W. K.Parker, one of the early students of 

 the embryology of the bird's skeleton, to declare that the 

 bird is " a transformed and, one might even say, a glorified 

 reptile." 



