218 THE DEVELOPMENT OF THE CHICK 



plate (see Figs. 128 and 129). The latter becomes perforated 

 after the eleventh day, and there is thus direct communication 

 between the albumen-sac and the amniotic cavity. Hirota 



Figs. 127, 128, and 129. — Diagrams of the relations of the embryonic mem- 

 branes of the chick, constructed from preparations, and from figures and 

 descriptions of Duval, Hans Virchow, Hirota and Fiilleborn. In these 

 figures the ectoderm and entoderm 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. 127 the allantois is represented as a sac. 

 In Figs. 128 and 129, 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 em- 

 bryo. The actual relations of the stalk are shown in Figures 33 and 82. 

 Alb., Albumen. Alb. S., Albumen-sac. All., Allantois. All. 1., Inner 



wall of the allantois. All. C, Cavity of allantois. All. S., Stalk of allantois. 



All. + Am., Fusion of allantois and amnion. Am., Amnion. Am. C, 



Amniotic cavity. Chor., Chorion. C. T. R., Connective tissue ring. Ect., 



Ectoderm. E. E. B. C, Extra-embryonic body-cavity. Ent., Entoderm. 



Mes., Mesoderm. S.-Am., Sero-amniotic connection. S. Y. S. U., Sac of 



the yolk-sac umbilicus. Umb., Umbilicus. V. M., Vitelline membrane. 



Y. S. S., Septa of the yolk-sac. 



Fig. 127. — 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 represented as a sac, the stalk of which enters the umbilicus 

 behind the yolk-stalk ; the allantois lies in the extra-embryonic body-cavity, 

 and its mesoblastic 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 undivided portion is slightly thickened to form 

 the beginning of the connective tissue ring that surrounds the yolk-sac 

 umbilicus. The ectoderm and entoderm 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 umV)ilicus. The albumen is not represented in this figure. 



Fig. 128. — Ninth day of incubation. The yolk-sac umbilicus has become 

 much narrowed; it is surrounded by the mesodermal connective tissue 

 ring, and by the free edges of the ectoderm and entoderm. The vitelline 

 meml)rane still covers the yolk-sac umbilicus and is folded into the albumen. 

 The allantois has expanded around the amnion and yolk-sac and its outer 

 wall is fused with the chorion. It has pushed a fold of the chorion over 

 the sero-amniotic connection, into which the mesoderm has penetrated, 

 and thus forms the upper fold of the albumen-sac. The lower fold of the 

 albumen-sac is likewise formed by a duplication of the chorion and allan- 

 tois; it must be understood that lateral folds are forming also, so that the 

 albumen is being surrounded from all sides. 



The stalk of the allantois is exaggerated so as to show the connection of 

 the allantois with the embryo; it is supposed to pass over the amnion, 

 and not through the cavity of the latter, of course. 



