STUDIES IN MAMMALIAN EMBRYOLOGY. 525 



Figs. 5 and 6. — Proliferation of the maternal epithelium having extended 

 over the surface opposite the mesometrium, the uterine wall has here again 

 thickened. The blastocyst has become attached to the maternal surface by 

 means of the omphaloidean trophoblast. 



Fig. 5.— Utr. Mas. Cat. n°- Sorex 73/, 2 r. 19 s. 



Fig. 6.— „ „ 45 c, 3 r. 14 s. 



Fig. 7. — First appearance of amnion, trophoblastic annulus and area vascu- 

 losa. Dehiscence in lateral maternal tissue, which has commenced in the 

 stage of Fig. 5, has here made rapid progress. 



Fig. 7.— Utr. Mus. Cat. n°- Sorex 42 e, 3 r. 11 5. 

 Figs. 8 and 9. — Proamnion and amnion simultaneously in process of forma- 

 tion. Secondary maternal epithelial crypts at their maximum of independent 

 development. First appearance of trophoblastic knobs on the allantoidean 

 trophoblast. Area vasculosa does not as yet extend down to trophoblastic 

 annulus. 



Fig. 8.— Utr. Mus. Cat. n°' Sorex 51 ^, 3 r. 29 s. 



Fig. 9.— „ „ 51 d, 2, 4 r. 2 s. 



Fig. 10. — Amnion closed and completed ; allantoidean trophoblast partly 

 applied against concavity of future placentary region ; trophoblastic knobs 

 just entering epithelial crypts. First appearance of allantois as free knob 

 projecting in extra-embryonic ccelom. Area vasculosa completed down to 

 trophoblastic annulus. Active participation of the latter in resorbtion of 

 hsemorrhagic extravasates consequent on disintegration of lateral tracts of 

 maternal tissue. 



Fig. 10.— Utr. Mus. Cat. n°- Sorex 106 e, 3 r. 9 s., and 106 h, 1 r. d s. 



Figs. 11 and 12. — The trophoblastic excrescences have penetrated into the 

 secondary epithelial crypts and have in their turn become hollowed out. 

 Villi of the allantois penetrating into these trophoblastic cavities. Epithelium 

 of maternal surface and crypts has disappeared wherever embryonic trophoblast 

 is in contact with it. In Fig. 11 the area vasculosa commences to loosen its 

 hold upon the lateral maternal surface ; in Fig. 12 it is hardly any longer 

 adherent. The actual distance between the trophoblastic annulus and the 

 placentary region has at the same time diminished. The embryo is being 

 sunk into the yolk-sac, thus pushing before it the area vasculosa, which is 

 thereby turned inside out. The differentiation of the allantoidean trophoblast 

 into a cytoblastic and a plasmodiblastic portion has commenced. In Fig. 12 

 the last traces of the ensheathing of the allantoidean villi by distinct cytoblast 

 are yet visible as a thin black line. 



Fig. 11.— Utr. Mus. Cat. n°- Sorex 3 i, 2, 1 r. 9 *. 

 Fig. 12.— „ „ 85 ^, 3 r. 1 s. 



Figs. 11 a and 11 b. — Two sections along a plane perpendicular to all the 

 foregoing. Same stage as that from which Fig. 11 was taken. The planes of 

 sections accompanied by the corresj)onding numbers are indicated in Fig. 11 

 by lines and numerals. In Fig 11 « only secondary crypts of maternal epithe- 



