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 
Fras. 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 Fiulleborn. 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 is 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. S8., Stalk of allantois. 
All. + Am., Fusion of allantois and amnion. Am., Amnion. Am. C., 
Amniotic cavity. Chor., Chorion. C. T. R., Connective tissue ring. Eet., 
Jetoderm. E. E. B. C., Extra-embryonie body-cavity. Ent., Entoderm. 
Mes., Mesoderm. S.-Am., Sero-amniotic connection. S. Y. 8S. U., Sae of 
the yolk-sae umbilicus. Umb., Umbilicus. V. M., Vitelline membrane. 
Y.S.8., Septa of the yolk-sac. 
Fic. 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-sae 
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 umbilicus. The albumen is not represented in this figure. 
Fig. 128. — Ninth day of incubation. The yolk-sae 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 
membrane 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-sae 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. 
