THE BLASTODERMIC VESICLE. 



23 



hollow sac is formed, walled by a single layer of cells, at ortt point on the inner sur- 

 face of which is attached a small mass of cells. The outer, covering layer of cells is 

 known as the tropJwblast ; the small group of cells attached to the inner surface of 

 the trophoblast is known as the inner cell-mass (Fig. 18). Examined from the sur- 

 face, this aggregation of inner cells appears as an opaque circular field, the embryonic 

 area, due to the increased thickness and consequently diminished transparency of the 

 wall of the blastodermic vesicle at the place of attachment of the included cells. In 

 the purest type of the blastodermic vesicle, that seen in the amphioxus (Fig. 26, 

 A), the sac consists of a single layer of blastomeres of almost uniform size ; the 

 mammalian blastodermic vesicle, however, presents greater complexity, due to the 

 unequal rate at which some of the segmentation cells divide and to the rapid increase 

 in the size of the vesicle. 



The inner mass of germinal cells soon undergoes differentiation (Fig. 19) into 

 two strata, — an outer layer, closely applied to the trophoblast, and an inner layer. 

 These layers are respectively the edoblast and the entoblast, — two of the three great 

 primary blastodermic layers from which the embryo is differentiated. 



Coincidently with the formation of these germinal layers, the mammalian blas- 

 todermic vesicle grows with great rapidity, increasing from a sphere of microscopic 

 size to a vesicle of one or more millimetres in diaineter. In consequence of this 

 growth, the trophoblast undergoes great expansion, its cells becoming reduced to 

 flattened elements, which, over the embryonic area, later disappear. In some ani- 

 mals, as in the rabbit, the flattened trophoblast cells extend over the embryonic 

 ectoblast and have been called the cells of Rauber. In such cases, therefore, the 

 ectoblast is overlaid within the embryonic area by the cells of Rauber, but at the 

 margin of the area, the embryonic ectoblast is continuous with the trophoblast form- 

 ing the outer layer of the wall of the blastodermic vesicle. ' With the subsequent 

 expansion of the blastodermic vesicle, the cells of Rauber disappear from the surface 

 of the embryonic ectoblast, which then hes upon the surface of the vesicle (Figs. 

 20, 21). 



Embryonic 

 ectoblast 



Mesoblast 



Etitoblast 



Trophoblast 



Diagram of mammalian blastodermic vesicle; 

 the entoblast forms an almost complete inner layer. 



Diagram of mammalian blastodermic vesicle ; the 

 mesoblast is just appearing as the third blastodermic 

 layer. 



The early blastodermic vesicle at first consists of only two primary layers, the 

 ectoblast and the entoblast ; this stage of development is appropriately termed that 

 of the bilatninar blastoderm (Fig. 20); a little later, a third layer, the mesoblast, 

 makes its appearance between the outer and inner blastodermic sheets ; this stage is 

 designated as that of the trilaminar blastoderm (Fig. 21). 



The early embryo, shortly after the formation of the blastodermic vesicle, con- 

 sists of three layers of cells,— the ectoblast, the mesoblast, and the entoblast. The 

 histological characters of the outer and inner of these primary layers differ, almost 



