BLASTULA, GASTRULA, AND GERM LAYERS 351 



embryo proper. In forms like the Elasmobranchs and 

 Sauropsids, the accumulation of yolk is still greater and the 

 embryo forms quite apart from the yolk, with which it later 

 acquires a secondary relation. In the Sauropsids, after the 

 rudiments of the embryo are well established, a process of fold- 

 ing begins and a series of infoldings of the cellular blastoderm, 

 anterior, posterior, and lateral, pinch off the embryo from the 

 yolk mass or yolk sac, with which it then remains only indirectly 

 connected by a narrow tube known as the yolk stalk which 

 includes a portion of the gut wall and a very abundant blood 

 supply. 



In the Sauropsids and Mammals other folds of the blastoderm soon 

 appear, beyond the limits of the embryo proper, which result in the 

 formation of a very special and highly characteristic structure known 

 as the cunnion. And from the wall of the hind-gut grows out another 

 special and extra-embryonic structure, the allantois. The formation 

 and function of these extra-embryonic structures, together called the 

 embryonic appendages, cannot be described here. They are of the 

 greatest importance in development and their presence has led to the 

 application of the term Amniota to all the forms possessing them 

 (Birds, Reptiles, Mammals) while the other Craniates, without these 

 embryonic appendages (Cyclostomes, Fish, Amphibia) are then known 

 as the An am ma. 



On account of the difficulties of comparison it seems wise to 

 omit reference here to the Mammalian gastrula and germ layer 

 formation. For in spite of the nearly alecithal condition of the 

 Mammalian ovum, its development shows marked yolk influ- 

 ence, and the whole course of early development is complicated, 

 not only through the one time presence and the subsequent loss 

 of yolk, but through the very special relations of the early 

 embryo, and particularly the embryonic appendages, with the 

 walls of the maternal cavity in which development proceeds. 



CONCRESCENCE 



We should consider here, in a particular way, a developmental 

 process which, besides being of great general importance in 

 Chordate development, is of considerable historical interest as 



