EXTRA-EMBRYONIC MEMBRANES 8 1 



During the third day a fold appears bounding the posterior 

 region of the embryo (Fig. 31, C). This caudal fold undercuts 

 the tail of the embryo forming a subcaudal pocket just as the 

 sub-cephalic fold undercuts the head. The combined effect of 

 the development of the sub-cephalic, lateral body, and the sub- 

 caudal folds is to constrict off the embryo more and more from 

 the yolk (Figs. 30 and 32). These folds which establish the 

 contour of the embryo indicate at the same time the boundary 

 between the tissues which are built into the body of the embryo, 

 and the so-called extra-embryonic tissues which serve temporary 

 purposes during development but are not incorporated in the 

 structure of the adult body. 



The Establishment of the Yolk-sac and the Delimitation of 

 the Embryonic Gut. The extra-embryonic membranes of the 

 chick are four in number, the yolk-sac, the amnion, the serosa 

 and the allantois. The yolk-sac is the first of these to make its 

 appearance. The splanchnopleure of the chick instead of 

 forming a closed gut, as happens in forms with little yolk, 

 grows over the yolk surface. The primitive gut has a cellular 

 wall dorsally only, while the yolk acts as a temporary floor 

 (Fig. 31, A). The extra-embryonic extension of the splanchno- 

 pleure eventually forms a sac-like investment for the yolk 

 (Figs. 30 -and 3 2). 



Concomitant with the spreading of the extra-embryonic 

 splanchnopleure about the yolk, the intra-embryonic splanchno- 

 pleure is undergoing a series of changes which result in the 

 establishment of a completely walled gut in the body of the 

 embryo. The interrelations of the various steps in the forma- 

 tion of the gut and of the yolk-sac make it necessary to repeat 

 some points and anticipate other points concerning the forma- 

 tion of the gut, in order that their relation to yolk-sac formation 

 may not be overlooked. 



It will be recalled that the first part of the primitive gut to 

 acquire a cellular floor is its cephalic region. The same folding 

 process by which the head is separated from the blastoderm 

 involves the entoderm of the gut. The part of the primitive 

 gut which acquires a floor as the sub-cephalic fold progresses 

 caudad is termed the fore-gut (Fig. 31, B). During the third 

 day of incubation the caudal fold undercuts the posterior end of 

 the embryo. The splanchnopleure of the gut is involved 



