STUDIES IN MAMMALIAN EMBEYOLOGY. 529 



intervening trophoblast (plasmodiblast) surrounding the blood-spaces (cf, Pigs. 

 13, 82, and 94). On the concave free surface of the placenta, however, the 

 cytoblastic trophoblast has attained to a maximum of thickness. It is here, 

 moreover, more or less honeycombed, incipient new allantoidean villi entering 

 into those recesses and being there attached and subsequently vascularised. In 

 this figure the connective tissue of the allantoic villi has been indicated, but the 

 spaces in these villi through which the embryonic blood-corpuscles circulate 

 have been left open (white). Two full-grown double villi, both of them 

 bifurcating, are here indicated, and three or four incipient ones. The tropho- 

 blastic layer, aii , being the continuation of the trophoblastic annulus (cf. Figs. 

 9 — 13, 50, 84, and Figs. 33 and 34), is closely applied against the allantois, 

 but does not in any way fuse with it. A uterine gland, with partly distended, 

 partly flattened lumen, is visible in this preparation. P. and T. as in 

 Fig. 29. 



Utr. Mus. Cat. n°- So rex 130 a, 3 r. 10 s. 

 Fig. 31. — A yet further stage of placentation. The trophoblastic region 

 in which the allantoidean villi are embedded occupies about five sixths of the 

 thickness of the wall ; the epithelial proliferation and gland-remains about 

 one sixth. Here, too, the embryonic blood-corpuscles are not indicated, and 

 the parts of the villi where they circulate remain white. The wide allan- 

 toidean vessel which supplies the three to four villi here represented is only 

 indicated in outline, as is also the lower boundary line of the allantois. The 

 maternal proliferation is yet more reduced, and of the crypts no distinct 

 remains persist (cf. Figs. 14 and 8G). 



Utr. Mus. Cat. n°- Sorex 61 a, 2 r. 3 s. 



Fig. 32. — A fragment of a section through the ripe placenta, perpendicular 

 to the surface, with low power (cf. Figs. 15 and 15 a). The spaces in which 

 embryonic and maternal blood-corpuscles circulate, although interwoven in a 

 most complex manner, can be distinguished {a) by the size of the blood- 

 corpuscles, which are very much smaller if maternal, (5) by the fact that in the 

 syncytium the maternal corpuscles circulate in spaces without any trace of 

 endothelium, whereas round the embryonic corpuscles the traces of the 

 endothelium of the allantoidean vessels (which in this figure are again repre- 

 sented by white spaces) are often preserved as flattened nuclei (cf. Figs. 

 52—54). 



Utr. Mus. Cat. n°- Sorex 100, 2 r. 12 s. 



Fig. 33. — The trophoblastic annulus, terminal portion of area vasculosa, 

 and downward membranous continuation by which the trophoblastic annulus 

 is connected with the placental border, in the stage of the uterus No. '^1 (cf. 

 Figs. 35 and 44). To the right of the membrane, ««', a transversely cut 

 allantoidean vessel is indicated (compare with Fig. 14). 

 Utr. Mus. Cat. n°- Sorex 77 a, 3 r. 4 s. 



Fig. 34. — The same for a yet later stage, viz. that of uterus No. 100. The 

 VOL. 35, PART 4. NEW SEK. 



