STUDIES IN MAMMALIAN EMBRYOLOGY. 395 
Tr. The trophoblast, in several parts more than one cell thick, between which 
a few free spaces (sp.) are visible. D. The decidual stroma. Jac. Distended 
blood-spaces in the immediate vicinity of the embryo. (Mus. Utr. Cat. 
n” Erin. 136 a, 3 7. 3s.) 
Fics. 8—10.—Somewhat older embryos, in which the polar knob (ep.) of 
the trophoblast (7.) is distinctly visible, out of which the epiblast of the 
germinal area is going to develop. Ay. Hypoblast. sp. Lacunar spaces in 
the trophoblast. 
In Fig. 8 (Mus. Utr. Cat. 1° Erin. 146 27, 6 s.) the very marked 
distinction between the embryonic and the peripheric hypoblast is 
clearly visible; in Fig. 8a the trophoblast is seen to surround the 
remnant of a uterine gland (cf. Fig. 39); in Fig. 10 (Mus. Utr. Cat. 
n° Erin. 1386 d,27. 7s.) the hypoblast has not the vesicular shape, 
but is a group of cells. In the space between it and the trophoblast, 
d represents a protoplasmic exsudate into the segmentation cavity, not 
separate nuclei. 
PLATE XVI. 
The figures on this Plate are intended to show the relative increase in size 
of the blastocyst in its different phases before the appearance of the mesoblast. 
They are drawn with the camera under the same power. 
Fies. 11 and 12 are similar to Figs 4 and 6 on the foregoing Plate. In 
Figs. 13 and 14 the polar knob in the trophoblast, from which the embryonic 
epiblast will develop, has made its appearance (14, ep.). In Fig. 15 it splits 
off from the trophobiast as represented under higher power in Fig. 15a. ep. 
Embryonic epiblast. ¢r. Trophoblast. ¢7. (a.) Trophoblast in the region 
which will afterwards become the placenta (allantoidean trophoblast). 
Fies. 16—20.—Further phases of this phenomenon. In Fig. 20 the meso- 
blast (here very diagrammatically indicated) has begun to be formed. In 
Figs. 18—20 the allantoidean trophoblast just mentioned, which is situated 
above the epiblast of the germinal area (ep.), is seen to have very considerably 
thinned out. This can be more definitely demonstrated by comparing Figs. 
15a@ (Mus. Utr. Cat. n™ Erin. 147 7,27. 13s.), 17a@ (Mus. Utr. Cat. nv 
Erin. 95, 27. 11 s.), and 204 with each other. In Fig. 204 (Mus. Utr. Cat. 
n” Erin. 272 ¢1, 47.75.) the attachment of the embryonic epiblast to the 
trophoblast takes place by means of the suspensory lamina (ep.). Fig. 20a is 
an enlarged figure of the thinned omphaloidean trophoblast of Fig. 20, with one 
of its cellular villiform projections. A comparison of these different figures 
with each other shows that in the stage of Fig. 14 (ef. Fig. 39) the relative 
maximum thickness of the trophoblast is reached. 
