THE FOSTAL MEMBRANES OF REPTILES AND BIRDS. 215 



Fig. 4. — Lmigitudijial section through a Hen's egg at the tegirming of the 



fifth day. 



After the fusion of the amniotic folds, the embryo, together with the amniotic 

 cavity {ah), is enveloped in the amniotic sac. The serous membrane {S) has been 

 developed from the outer layer of the amniotic folds. By further separation 

 of the middle germ-layers the extra-embryonio part of the body-cavity (W) 

 has enlarged, and the allantois {al) has grown into it. 



With the exception of a third of its surface, the yolk has become overgrown 

 by the outer and inner germ-layers, as far as the line ur. The vascular area 

 has extended to the line St. The cephalic portion of the intestinal cavity has 

 opened into the amniotic cavity by means of the newly arisen mouth (m). 



Fig. 5. — Longitudinal section through a Hen's egg on the seventh day of 

 incubation. 



By the enlargement of the extra-embryonic body-cavity the serous membrane 

 (serosa) has entirely separated from the yolk-sac, with the exception of a small 

 area. The outer and the inner germ-layers have now grown over the yolk on all 

 sides ; the middle germ-layer with the vascular area has extended farther 

 downwards. The amniotic cavity, in which the embryo floats, has become 

 much extended by the increase of the amniotic fluid. The allantois has 

 enlarged considerably, and forms a sac, which connects with the hind gut 

 by means of a narrow stalk (urachus). The sac extends out into the extra- 

 embryonic body-cavity between amnion, yolk-sac, and serous membrane, more 

 particularly on the right side of the embryo. 



Fig. 6 represents a diagrammatic cross section through an emiryo Fish. 



The dorsal part is already far advanced in development and encloses the 

 neural tube (iV), the chorda (ch), the aorta (ao), and the primitive segments. 

 The ventral side is greatly distended by the considerable yolk-mass (d). The 

 latter lies in an enlargement of the intestinal canal, the intestinal yolk-sac ; 

 this is separated from the enlarged dermal yolk-sac by means of a narrow 

 fissure, the body-cavity {Ih). 



Fig. 7. — Diagrammatic longitudinal section through a Selachian embryo. 



The yolk-sac has been partly constricted off from the body of the emoryo ; 

 it still remains united to its ventral sidte, but only by means of a narrow stalk (si), 

 which consists of two tubes, one within the other, the intestinal stalk (vitelline 

 duct) and the dermal stalk. Tie yolk-sac communicates with the embryonic 

 intestinal canal by means of the vitelline duct. The point of transition is 

 called the intestinal umbilicus (dn). The point of attachment of the dermal 

 stalk to the belly of the embryo is the dermal umbilicus (hn). The space 

 between dermal and intestinal umbilicus (Are and dn) serves to put the body 

 cavity of the embryo (Z/j') in communication with the body-space (Ih^) between 

 the dermal and intestinal yolk-sacs. 



