OUI'LINES OF THE DEVELOPMENT OF THE TUATAEA. 23 



amniotic membrane thus formed around the head becomes 

 completely separated from the overlying blastoderm (area 

 pellucida), which lies at some little distance above, and exhibits 

 no special peculiarities (fig. 23). 



As we trace the pro-amnion backwards we find that at the 

 posterior limit of the anterior closure of the alimentary canal 

 its inner layer becomes continuous with the epiblast of the 

 embryo proper, while its outer layer passes into the hypo- 

 blastic lining of the alimentary canal (fig. 25). A little further 

 back the outer or hypoblastic layer of the pro-amnion becomes 

 continuous above with the ill-defined hypoblast of the over- 

 lying blastoderm, to which the embryo is thus attached in the 

 mid-dorsal line (fig. 27). Still further back the amnion is 

 formed by an uprising of the epiblast on each side of the mid- 

 dorsal line, forming two folds which meet and fuse above the 

 medullary groove, and thus arch it over, while mesoblast cells 

 extend in between the two layers of the epiblast (figs. 28, 29). 

 Further back again the arching over of the medullary groove by 

 the amnion is as yet incomplete, so that the narrow amniotic 

 cavity opens widely to the exterior (fig. 30), and behind this 

 the amnion has not yet begun to develop (figs. 31, 32). 



In embryo 64 the formation of the amnion has extended 

 back as far as the region of the primitive streak, which, how- 

 ever, is still incompletely arched over by the uprising folds of 

 the somatopleure (fig. 34). 



A comparison of transverse sections through the posterior 

 portions of embryos of Stages C and D shows that the formation 

 of the amnion is here also accompanied by a downsinking of 

 the body of the embryo, but not nearly so marked as in the 

 anterior part (compare figs. 9 — 14 with figs. 22 — 32). 



The separation of the serous envelope, commenced already 

 in the preceding stage, is progressing in the hinder part of the 

 area pellucida. Figs. 29 — 32 show clearly that it consists of 

 the superficial epiblast of the area pellucida, with a few 

 adherent irregular mesoblast cells (derived from the original 

 lower layer), which splits off from the deeper mesoblast derived 

 partly from outgrowth of the primitive streak. 



