THIi PLAOENTATION OF PEBAMELES. 399 



Tlie ectoderm (fi"^. 11, ect.) is markedly thickened, and at 

 the margins of the area forms cushions several cells in thick- 

 ness. Tiie superficial cells often project freely, and are club- 

 like in form, witii the nucleus lying in the freely projecting 

 part of the cell. Apparently in Phascolarctus such a marked 

 thickening of the ectoderm does not exist. 



The entoderm presents a somewhat varied appearance in 

 different sections. In the section drawn (fig. 14, ent.) it is a 

 quite irregular layer of some thickness. Many of its cells are 

 greatly enlarged and vesicular-looking, presenting quite a 

 degenerate appearance. Even in places where the entoderm 

 over the area does not differ greatly from the entoderm of the 

 yolk splanchnopleure, one often meets with similar isolated 

 enlarged and vesicular-looking cells. In Phascolarctus Semon 

 describes the entoderm over the area as "stark verdickt und 

 eigenthiimlicli gewulstet^' (8, p. 31). At the edges of the area 

 (fig. 14) the somatic mesoderm of the amnion {amn.) is con- 

 tinuous with the mesoderm of the splanchnopleure, so that the 

 continuity of the extra-embryonic coelom is here definitely 

 interrupted. The mesoderm penetrates into the area for a 

 short distance peripherally ; but, contrary to what Semon 

 describes for Phascolarctus, it does not form a continuous 

 layer extending right through and separating the ectoderm of 

 the amnion from the entoderm of the yolk-sac. In the centre 

 of the area these layers may be either in close apposition or 

 separated by a narrow cleft. This central portion of the area 

 thus consists of true proamnion. 



Stage C. — P. obesula. 



The left uterus was very much larger than the somewhat 

 enlarged right, and formed a large globular swelling containing 

 a single embryo (fig. 5) with a crown-rump measurement of 

 7 mm. For details of the internal anatomy of this embryo 

 see table (appendix). 



This is a most important stage, since it shows the mode of 

 attachment of the embryo. Microscopic examination of the 



VOL. 40, PART 3. NEW SEB. E E 



