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From Embryology

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Figure 5. Diagram to illustrate the condition of Entypy of the the Germinal Area

The trophoblast is represented by continuous black lines or masses, there entoderm by interrupted lines, the embryonic ectoderm, and in certain figures the amniotic ectoderm, by epithelial cells.

Each figure represents the blastodemic 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 rnbbit (a) the cells of the embryonic knob early become arranged as an oiiithulial plate at the upjier pole of the blastocyst : the covering layer of trophoblaat (Rauber's layer) disappears and it is exposed on the surface— there is no amnio-embryonic cavity.

in Ihe mole (b) the embryonic plate is for a short time ioturned. 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 kaah. 'I'hu tliHir of this forma the embryonic ectoilerm which is necessarily at IJrst coni«vo (f'.f. Intumeil) ; the roof pereists as the primilive amnion.

In the mouse or rat [d) the covering layer of tmphoblast roo5ng in the primitive niiiiilotii! unvity Iwconies greatly thickened, and ihe slightly intumed embi^onie plate (a jiuBhiMl Inwards as the blastocyst elongates into a tubular shape, until the layiTH (i-i'tiHlerm and ontD<lemi) arc apparently reversed in position.

In the guinea-pig (e) the same inpushing occurs, but the amnio-embryonic rudiment and the trophoblastic thJckciimg arc separated ; the amnio-embryonic vesicle remaiiiH a olosiil vesicle iiud forms the definitirr amniotic cavity.

In Pteropus (f) the conditions are much the same as in Cavia, bat the blastocyst remains rounded and there is no greater intuming of the layers than occurs in the early phases of the mole.

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