THE BLASTODERMIC VESICLE. 



61 



The point where the inner mass and the subzonal layers are connected with 

 one another marks the site of the future embryonic area. 



The blastodermic vesicle grows rapidly in size, partly by the multiplication 

 of its cells, partly by their becoming flattened out so as to cover a larger surface. 

 The interior of the vesicle is filled with fluid. As the vesicle grows the fluid in- 

 creases in amount, and is presumably 

 derived by the ovum from the walls of 

 the uterus. It is under pressure within 

 the vesicle, as is shown by the manner 

 in which it spurts out if the vesicle is 

 broken. Nothing exact as to the com- 

 position of this fluid is known, though 

 we may suppose it to resemble more or 

 less the serous fluids of the adult body. 

 The size and form of the vesicle offer 

 characteristic variations in mammals. 

 It starts as a more or less nearly 

 spherical body. In the rabbit it as- 

 sumes an oval shape, and by the seventh 

 day measures about 4.0 mm., and soon 

 thereafter becomes attached to the wall 

 of the uterus. In the hedgehog, the 

 guinea-pig, and the mouse the ovum, 

 while very small and more or less 

 rounded in form, becomes imbedded in 

 uterine tissue and develops into a 

 special shape in adaptation to its new 

 situation. In the ungulates the vesicle 

 grows enormously, becoming a very 

 long and slender sack. Thus, for ex- 

 ample, in the sheep it may measure 

 on the fourteenth day not less than 50 

 cm. in length. 



Another respect in which the blastodermic vesicles differ greatly from one 

 another in various mammals is in regard to the early development of the sub- 

 zonal layer, or, as we may call it, the ectoderm. In many cases the entire layer 

 undergoes a precocious development, its cells multiply very rapidly, so that the 

 layer becomes several cells thick. This thickened layer is known as the tropho- 

 blast. In other placental mammals this thickening is confined to a limited area 

 of the ectoderm. For further description see Trophohlast. 



Fig. 17. — Sections through the Inner Mass 



OF Blastodermic Vesicles of the Mole 



AT Three Successive Stages. 

 Ec, Outer or subzonal layer, z, x. Zona pellucid a. 



t.m, Inner mass of cells. /ly. Entoderm. — 



{After IV. Heape.) 



