THE EARLY DEVELOPMENT OF THE CHICK 287 



surface of the yolk. Throughout its pellucid and vascular areas 

 it is divided by the exocoelom, into somatopleure and splanchno- 

 pleure (Fig. 108). Its outlying area vitellina for the most part 



All.} 



Umb. 



-Eel. 



Mes 



FIGS. 115, 116, 117. Diagrams of the relations of the embryonic membranes 

 in the chick. Figures and description from Lillie (Development of the Chick). 

 The ectoderm and endoderm are represented by plain lines; the mesoderm by 

 a cross-hatched line or band. The yolk-sac is represented by broken parallel 

 lines. In Fig. 115 the allantois is represented as a sac. In Figs. 116 and 117, 

 where it is supposed to be seen in section, its cavity is represented by unbroken 

 parallel lines. The stalk of the allantois is exaggerated in all the diagrams to 

 bring out its connection with the embryo. 



FIG. 1 15. Fourth day of incubation. The embryo is surrounded by the amnion 

 which arises from the somatic umbilicus in front and behind; the sero-amniotic 

 connection is represented above the tail of the embryo; it consists at this time 

 of a fusion of the ectoderm of the amnion and chorion. The allantois is repre- 

 sented as a sac, the stalk of which enters the umbilicus behind the yolk-stalk; 

 the allantois lies in the extra-embryonic body-cavity (exocoelom), and its meso- 

 dermal layer is fused with the corresponding layer of the chorion above the embryo. 

 The septa of the yolk-sac are represented at an early stage. The splitting of 

 the mesoderm has progressed beyond the equator of the yolk-sac, and the un- 

 divided portion is slightly thickened to form the beginning of the connective- 

 tissue ring that surrounds the yolk-sac umbilicus. The ectoderm and endoderm 

 meet in the zone of junction, beyond which the ectoderm is continued a short 

 distance. The vitelline membrane is ruptured, but still covers the yolk in the 

 neighborhood of the yolk-sac umbilicus. The albumen is not represented in 

 this figure. (For explanation of lettering see Fig. 117.) 



consists of ectoderm and endoderm only, and is bounded per- 

 ipherally by a region where the further extension of the blasto- 

 derm is being effected. The extension of the blastoderm con- 



