THE GERM-LAYERS. 13 



the surface of the blastodermic vesicle presents an opaque circular field, the 

 embryonic area, so called from the fact that within this area the first traces 

 of the future embryo appear. 



In consequence of further growth and differentiation of the inner cell- 

 mass, the latter gives rise to two sheets of cells, the ectoderm and the 

 entoderm. The first of these is continuous with the trophoblast and, in con- 

 junction with the latter, completes the outer layer of the blastodermic vesi- 



FiG. 10. Early stages of segmentation as seen in sections of ova of mouse. X 45- (Sobotta.) 

 A-D show the rearrangement of the chromosomes contributed by the male (i) and female (f) germ- 

 cells as preparatory to the first cleavage of the fertilized ovum ; /, /, polar bodies ; e p, stage of equatorial 

 plate ; a, b, daughter groups of chromosomes. , f, the daughter cells arising from first cleavage. G, 

 one cell (6) is larger and is preparing to divide. //, later stage of this division. /, stage of three seg- 

 mentation spheres (a and c, c) resulting from this division. 



cle. The entoderm gradually expands until it forms a complete second 

 layer within and concentric with the outer stratum of the blastodermic wall. 

 Meanwhile a third layer of cells, the mesoderm, makes its appearance be- 

 tween the ectoderm and the entoderm and, in time, converts the wall of the 

 blastodermic vesicle into a trilaminar envelope. The three cell-sheets 

 derived from the inner cell-mass constitute the blastodermic or germ-layers 

 structures of great importance, since they supply the cells from which all 

 parts of the embryo are developed. The histological characters of the outer 

 and inner of these primary layers differ, almost from the first, from those of 

 the mesoderm, their component elements being more compact in arrange- 

 ment and early acquiring the characteristics of covering cells or epithelium. 



