32 
AN EARLY HUMAN OVUM 
Tarsius, before the embryonic ectoderm is differentiated from the amniotic. 
Owing probably to the relatively great expansion of the trophoblast 
Figure V. Diagrams to illustrate the condition of Entypy of the Germinal 
Area (T. H. Bryce, from “Quain’s Anatomy,” 11th Ed. Vol. I.). 
The trophoblast is represented by continuous black lines or masses, the ento¬ 
derm by interrupted lines, the embryonic ectoderm, and in certain figures the 
amniotic ectoderm by epithelial cells. 
Each figure represents the blastodermic vesicle, a, of the rabbit; b, of the 
mole ; c, of the bat; d, of the mouse or rat; e, of the guinea-pig ; f, of the kalong 
(Pteropus edulis). 
In the rabbit (a) the cells of the embryonic knob early become arranged as an 
epithelial plate at the upper pole of the blastocyst; the covering layer of tropho¬ 
blast (Rauber’s layer) disappears and it is exposed on the surface—there is no 
amnio-embryonic cavity. 
In the mole (b) the embryonic plate is for a short time inturned. The hollow is 
filled with trophoblast cells which disappear, and the plate straightening out is 
exposed on the surface as in the rabbit. 
In the bat (c) a more distinct cavity appears in the heart of the embryonic knob. 
The floor of this forms the embryonic ectoderm which is necessarily at first con¬ 
cave (i.e. inturned); the roof persists as the primitive amnion. 
In the mouse or rat (d) the covering layer of trophoblast roofing in the primitive 
amniotic cavity becomes greatly thickened, and the slightly inturned embryonic 
plate is pushed inwards as the blastocyst elongates into a tubular shape, until the 
layers (ectoderm and entoderm) are apparently reversed in position. 
In the guinea-pig (e) the same inpushing occurs, but the amnio-embryonic rudi¬ 
ment and the trophoblastic thickening are separated ; the amnio-embryonic vesicle 
remains a closed vesicle and forms the definitive amniotic cavity. 
In Pteropus (/) the conditions are much the same as in Cavia, but the blasto¬ 
cyst remains rounded and there is no greater inturning of the layers than occurs in 
the early phases of the mole. 
shell, and tardy differentiation of the embryonic rudiment, the entoderm 
does not grow round the blastocyst wall, but takes the form of a small 
closed vesicle, the cavity of which very possibly appears within a solid 
