GERM-AKEA. 



293 



the contrary, it arises from the outer of the two ; ^^ both are 

 probably concerned in the process. The middle of the 

 germ-area, or germ-disc, now consists of three germ-layers, 

 Avhile the circular rim consists of two ; the rest of the wall 

 of the intestinal germ- vesicle consists only of a single germ- 

 layer, the outer. But this wall also now becomes two-layered. 



Fig. 76. — Section through, the germ. area of a Mammal, at right angles 

 to the surface (diagrammatic) : e, exoderm (the simple cell-stratum of the 

 skin-layer) ; m, mesoderm (the several cell-stratum of the middle layer) ; 

 c, entoderm (the simple cell-stratum of the intestinal layer) ; A-, hollow 

 cavity in the intestinal germ.vesicle. 



While, in the centre of the germ-area, the fibrous layer 

 becomes greatly thickened, in consequence of cell-growth, 

 the inner germ-layer simultaneously extends and grows in 

 all directions from the edge of the disc. Everywhere closely 

 applied to the outer germ-layer, it completely overgrows the 

 inner surface of the latter ; it covers first the upper, and 

 then the lower hemisphere of the inner surface, and finally 

 closes in the centre of the latter. (Cf Fig. 77-81.) The 

 whole wall of the intestinal germ-vesicle now, therefore, 

 consists of two cell-strata: the exoderm without, the entoderm 

 within. In the centre only of the circular germ-disc, which, 

 in consequence of the excessive growth of the middle 

 layer, continually increases in thickness, this germ-disc 

 consists of all three germ-layers. Simultaneously, small 

 structureless knots, or warts, secrete themselves on the 

 surface of the outer egg-membrane [iwochorion), which 



