THE DEVELOPMENT OF VERTEBRATA. 257 



9. Embryo and Yolk-sac. From about the end of the 



first day of incubation we may trace the gradual progress 

 of a process by which the embryo becomes more and more 

 sharply delimited from the extra-embryonic structures, and 

 at the same time assumes more and more its proper shape 

 instead of the flattened-out condition, devoid of any ventral 

 surface, with which it starts. This is essentially a process 

 of pinching-ojf of the embryo from the yolk. To get a first 

 notion of it imagine a balloon with a rope passed round its 

 middle : let the rope be drawn tighter and tighter, so as to 

 gradually nip the balloon into two portions, separated by a 

 narrow neck. The upper portion answers to the embryo, 

 the lower to the yolk-sac, and the narrow neck to the stalk 

 of the yolk-sac. 



This first crude notion must be modified in several ways 

 before a correct idea is obtained. In the first place, the 

 embryo, when first its margin is pinched in, is vastly smaller 

 than the yolk, and while the pinching-in is proceeding it is 

 constantly growing larger ; and as it gets most of its food- 

 material from the yolk (some coming from the albumen), 

 in proportion as the embryo grows larger the yolk-sac gets 

 smaller. This change in relative size must therefore be 

 borne in mind when the pinching process is imaged mentally. 



A much more important divergence from the simple 

 process of pinching is the simultaneous formation of the 

 amnion a protective covering to the embryo. In figs. 

 131 to 136 an attempt has been made to show by dia- 

 grammatic transverse and longitudinal sections, the gradual 

 change in the relation of embryo and extra-embryonic 

 structures. These diagrams represent the embryo as far 

 larger proportionately to the yolk than it really is at any of 

 these stages. They show the gradual extension of the 

 blastoderm (area opaca) around the yolk, and how the split- 

 ting of the mesoblast follows round, until the yolk becomes 

 completely enclosed in a yolk-sac consisting of hypoblast and 

 splanchnic mesoblast. They also show the gradual pinching- 

 in by which the mesenteron becomes more and more com- 

 pletely separated from the yolk-sac, until a well-defined stalk 

 alone unites them. They further show how the epiblast and 

 somatic mesoblast are also tucked in ventrally to form a 

 ZOOL. 17 



