110 



TEXT-BOOK OF EMBRYOLOGY. 



body (Fig. 100, 2). The folds a mniotic folds appear cranially, laterally and 

 caudally. These folds continue to grow dorsally (Fig. 100, 3) and finally meet 

 and fuse above the embryo (Fig. 100, 4)- They then break through along the 

 line of fusion so that the extraembryonic body cavity which has been carried up 

 dorsally over the embryo in the amniotic folds becomes continuous across the 

 mid-dorsal line. A double membrane or rather two membranes are thus 

 formed which extend over the embryo. The outer membrane is the chorion 

 and is composed from without inward of ectoderm and parietal mesoderm. 

 The inner membrane is the amnion and is composed from without inward of 

 parietal mesoderm and ectoderm (Fig. 100, 5). Between the amnion and the 

 chorion is a portion of the extraembryonic body cavity, which, as already 

 mentioned, was carried dorsally with the amniotic folds (Fig. 100, 2, 3, 4 and 5). 



Sclerotome Myotome 



Entoderm 



Pronephric 

 tubule 



FIG. 101. Transverse section of a dog embryo with 19 primitive segments. Bonnet. 

 Section taken through sixth segment. 



In the manner just described the amnion becomes a sac which at first en- 

 closes the embryo laterally, and then laterally and dorsally (Fig. 101). Later 

 as the embryo becomes constricted off from the underlying cavity, the amnion 

 encloses it entirely except over a small area on the ventral side where the embryo 

 is attached to the yolk sac (Fig. 100, 3, 4 and 5). 



While the amnion is being formed, the mesoderm continues to extend 

 around the vesicle between the ectoderm and the entoderm. At the same time 

 it splits into parietal and visceral layers, of which the parietal is applied to the 

 ectoderm, and the visceral to the entoderm. In this way the extraembryonic 

 body cavity gradually extends farther and farther around the vesicle until 

 finally the somatopleure is completely separated from the splanchnopleure 

 (Fig. 100, 3, 4 and 5). The extraembryonic somatopleure now forms a com- 

 plete wall for the vesicle and constitutes the chorion. The extraembryonic 

 splanchnopleure forms a complete wall for the yolk cavity and constitutes the 

 wall of the yolk sac. The proximal portion of the yolk sac becomes constricted 



