84 EDWARD MCCRADY, JR. 



33, is equally 'cephalic;' however, the classical nomenclature 

 here is probably too well established to be displaced. 



This flexure in the opossum occurs at a time when closure 

 of the neural folds has not progressed anterior to the myelen- 

 cephalon (fig. 28, reconstruction and sections A, B, C, D). 



Beginning of amnion formation. It will be recalled that 

 there has been no Rauber's layer in the opossum at any 

 time. The cells from which the surface of the embryonic body 

 is formed have been superficial from the beginning. Also the 

 vesicle has not formed any association with the endometrium, 

 but floats freely in the 'uterine milk.' Amnion formation, 

 therefore, is not complicated by the presence of any extrane- 

 ous material, or by any implantation process. 



The homology of the unilaminar and bilaminar blastocysts 

 with the blastula and gastrula of the lower vertebrates has 

 already been commented upon. The entire external lining of 

 the vesicle is ectodermal. The so-called trophoblast is merely 

 the primitive ventral ectoderm. The internal lining of the 

 vesicle is entirely endodermal, at first corresponding to the 

 archenteron, and later to the enteron plus the yolk sac. When 

 mesoderm forms at the animal pole, it separates ectoderm and 

 endoderm. As it spreads, it extends this separation through- 

 out one-third to one-half of the vesicle; but it never invades 

 the ventral half. Along an elongate, elliptical line enclosing 

 the animal pole the mesoderm splits tangentially to form the 

 somatopleuric and splanchnopleuric layers with the coelomic 

 space between them. Part of each of these layers is to be 

 incorporated into the embryonic body and part to be dis- 

 carded, but there is no visible distinction between the embry- 

 onic and accessory cells. Finally, the most dorsal mesoderm, 

 which is within this ellipse, is again unsplit. 



At stage 26, therefore, the original bilaminar vesicle re- 

 mains unmodified in its ventral half to two-thirds. The next 

 most dorsal portion including the layer of unsplit mesoderm 

 is the area vasculosa or vascular portion of the yolk sac. The 

 definitive embryonic body is developing approximately at the 

 center of this vascular area, and the coelom is confined to a 

 narrow elliptical strip in the immediate vicinity of the embryo. 



