STUDIES IN MAMMALIAN EMBRYOLOGY. 537 



tissues. Blood circulates in the maternal proliferation in capillaries that 

 generally have a distinct endothelium. In the trophoblast it circulates in 

 intra- and intercellular cavities. Two allantoidean villi are here only indi- 

 cated by their embryonic blood-corpuscles. Between these villi the cyto- 

 blastic and plasmodiblastic portion of the trophoblast is situated, the two 

 having become artificially separated from each other as in Eig. 81. It is 

 impossible to distinguish between those cells that have a maternal origin and 

 those that are derived from the embryo, otherwise than genetically. 



Utr. Mus. Cat. n°- Sorex 85/, 3 r. 15 s. 

 Figs. 90 — 92. — Three portions of the allantoidean trophoblast taken from 

 one and the same series of sections of uterus No. 101 (Fig. 36). These 

 sections indicate how the allantoidean trophoblast proliferates and is trans- 

 formed into intercommunicating blood cavities, nuclear nests breaking away 

 from the outer layers of the syncytium, and arranging themselves into blood 

 channels by processes of stretching and hollowing out. In principle it is this 

 same process by which, in the earlier phases such as that of Figs. 79, 80, and 

 93, the trophoblast has developed and become the peculiar sanguiniferous 

 interstitial tissue between the villi. 



Fig. 90.— Utr. Mus. Cat. n°- Sorex 101 a, 1, d r. 5 s. 



Fig. 91.— „ „ 101 a, 1, 1 r. 10 s. 



Fig, 92.— „ „ 101 a, 1, 2 r. 10 s. 



Fig. 93. — The same in an earlier stage (uterus No. 85, Fig. 40 ; cf. also 

 Figs. 12, 29, 81, and 89). The darker layers of cytoblast have the somewhat 

 more faintly stained plasmodiblast between them. The latter is in this stage 

 seen to be split off from the former in parallel layers that develop blood- 

 spaces between them (cf. Figs. 29, 30, and 89). The cytoblast at the top of 

 the figure belongs to a cavity in which an allantoidean villus is situated (cf. 

 Fig. 29, vphich represents the same stage). 

 Utr. Mus. Cat. n°. Sorex 85 a, 2 r. 23 s. 



Fig. 94.— The same, one stage later (cf. Figs. 13, 30, and 82) at the 

 period that the concave layer of cytoblast has attained its maximum of 

 development. Here, too, the newly-formed blood-spaces are primarily 

 arranged in a more lamelliform way close to the inner layer, whereas higher 

 up between the villi this vasifactive plasmodiblast is already more like that of 

 Figs. 90—92. Indentations on the lower surface for the reception of secondary 

 or tertiary allantoidean villi. 



Utr. Mus. Cat. n°' Sorex 130 a, 3 r. 10 s. 



