THE EMBRYOLOGY OF THE OPOSSUM 115 



mesoderm splits to form somatic and splanchnic layers with 

 all the blood vessels outside the body in the lower of the two 

 the splanchnic layer (fig. 39, reconstruction). This leaves the 

 surface of the vesicle non- vascular in the immediate vicinity 

 of the embryo. I have already described how the mesoderm 

 becomes resorbed in the proamniotic region, and how the head 

 elongates, reaches over the coelomic loop and dips ventrally 

 into the proamnion, thus forming the head fold of the aninion 

 of only ectoderm and endoderm. When the caudal and lateral 

 folds arise (stage 27) they are formed from the roof of the 

 extra-embryonic coelom. Accordingly when they meet above 

 the embryo and fuse, they form a non-vascular somato- 

 pleuric spot in the midst of the area vasculosa. This sheet of 

 somatopleure underlain by extra-embryonic coelom, though of 

 small area, is a permanent part of the outside wall of the 

 vesicle and is of different constitution from the other two 

 parts already described. This is the serosal chorion (fig. 40, 

 s.ch., and fig. 39, reconstruction). 



There are thus three parts of the definitive chorion in the 

 opossum: 1) the non-vascular yolk sac chorion ectoderm 

 and endoderm; 2) the vascular yolk sac chorion ectoderm, 

 mesoderm, and endoderm; 3) the serosal chorion ectoderm 

 and somatic mesoderm. This last is the spot at which an 

 allantoic placenta (such as is characteristic of all Eutheria) 

 could, theoretically, form, if the allantois, which projects into 

 the extra-embryonic coelom, should grow out caudally far 

 enough to reach and fuse with the outside wall of the vesicle. 

 But this does not occur in the opossum (see stage 33). 



The principal peculiarities which give rise to this type of 

 chorion are 1) the mesoderm does not spread more than 

 one-third of the way around the vesicle; 2) the process of 

 splitting into somatic and splanchnic layers with a coelom 

 between them occurs in an exceedingly restricted region 

 around the embryo proper; 3) as a consequence of the second, 

 the yolk sac remains a permanent constituent of the outside 

 wall of the vesicle throughout most of its area; and 4) the 



