LATER DEVELOPMENT OF CHICK AND RABBIT 419 



covered save for a long oval opening towards the hinder end. This 

 gradually narrows down, and is completely closed up by the end of 

 the third day of incubation. As the folds meet over the embryo the 

 junction between them breaks down save over a limited area, where 

 it persists, and their constituent layers fuse. Thus it comes about 

 that the embryo becomes covered superficially by two very thin 

 membranes. The innermost of these is the amnion, and it is com- 

 posed of a layer of ectoderm continuous with the embryonal ectoderm 

 on the inside and a layer of somatic mesoderm on the outside. The 

 embryo itself then comes to lie in the floor of a hollow cavity, the 



LA 



EC 



Sp 



FIG. 147. Chick embryo. Transverse section of embryo with twenty-eight 

 pairs of somites (about 48 hours) in region where grit remains open.- 

 Adapted from Duval. 



D.A., dorsal aorta ; E., embryonic coelom ; EC., ectoderm ; En., entoderm ; Ex., exocoel; 

 G., enteric groove; L.A., lateral amniotic folds; N., notochord ; S.M., somatic mesoderm; 

 Sp.C., spinal cord ; Sp.M., splanchnic mesoderm ; V., vitelline veins. 



amniotic cavity, which becomes filled with a clear liquid, the amniotic 

 fluid. Outside this, and separated from it by an extension of the 

 extra-embryonic coelom, is the second layer, the chorion or false 

 amnion. This is composed of the somatic mesoderm, internally and 

 externally, by ectoderm continuous with the general extra-embryonic 

 ectoderm. It has been noted that along the line marking the final 

 point of closure of the amniotic folds the junction between them does 

 not break down. The folds remain firmly united in this region, 

 which is termed the sero-amniotic connection, and this affects the 

 future arrangements of the membranes to a considerable extent. 



