THE I'LACENTATION OF PERAMEI-ES. 415 



SO close, iudeedj is the attachment that the walls of the capil- 

 laries appear as if united with the syncytial protoplasm. The 

 capillaries dip down into and accurately fill up the depressions 

 between the vascular ridges, so that there is here and there 

 formed an actually interlocking system of vascular projections 

 of the syncytial and allantoic surfaces respectively (figs. 16 

 and 17). As already indicated, this interlocking does not 

 occur uniformly all over the placental area, but varies in its 

 degree of perfection in different places (cf., e. g., figs. 15, 18, 

 and 19 with fig. 17). The interlocking here referred to repre- 

 sents the highest state of placental differentiation realised in 

 Perameles. 



It is thus evident that in this placental differentiation foetal 

 and maternal elements take an approximately equal share. In 

 the functional organ so produced, it will be noted that the 

 fcetal and maternal blood-streams are separated from each 

 other only by the thickness of two endothelial walls, with at 

 most the addition of a thin layer of syncytial protoplasm. 

 We may here point out that in Perameles, contrary to what 

 obtains in most other mammals,^ the uterine glands of the 

 placental area do not degenerate, but persist throughout the 

 whole period of pregnancy. In this stage some of the gland 

 openings in the placental area are still occluded by persistent 

 portions of the chorionic ectoderm in a more or less degenerate 

 condition ; others of the openings, however, are obstructed by 

 allantoic capillaries extending right over them (fig. 18,^/.). 

 It may be that nutritive substances derived from the utei'ine 

 secretion of such glands are directly absorbed by the allantoic 

 capillaries occluding their openings. 



(c) Yolk Splanchnopleure. — This is essentially as in 

 Stage C. We may mention, however, that the yolk-sac cavity 

 no longer stands in open communication with the gut, as was 

 the case in that stage. 



(d) Vascular Omphalopleure and Yolk-sac Placenta. 



' Strahl (15) and Veruhout (14) describe a similar persistence of the uterine 

 glands in the mole. According to Veruhout they are invaded by the " plas- 

 modiotrophoblast " shortly before parturition. 



VOL. 40, PART 3. — NEW SER. P P 



