THE PRIMITIVE AXIS. 65 



walls just outside the edge of the shield (Fig. 20), while in the region of the shield 

 there is no middle layer whatever. By the time the primitive streak has ap- 

 peared in the sheep, the formation of the mesoderm has extended under the em- 

 bryonic shield, and the relations between the germ-layers then become essen- 

 tially as above described. 



The cells of the mesoderm are at first quite closely packed, but as the layer 

 grows they begin to move apart, though remaining connected with one another 

 by protoplasmic processes. The moving apart of the cells is least near the primi- 

 tive streak and becomes more marked as we go toward the periphery of the layer 

 (Fig. 21 ), which represents a part of the peripheral region of the mesoderm of a 

 blastodermic vesicle of a rabbit of seven days. 



In the details of its expansion the mesoderm varies greatly in different 

 mammals. In some forms it develops very early and rapidly expands over the 

 entire blastodermic vesicle, which then becomes three-layered. This seems to 

 be the method of its growth in man and other primates. In other cases, as in the 

 dog and cat, it grows more slowly, but ultimately encloses the entire entoderm. 

 In the rabbit, on the contrary, it never expands more than about three-fifths of 

 the way over the blastodermic vesicle, one part of which, therefore, — viz., that 

 opposite the embryo, — never has any mesoderm whatever. This, however, is to 

 be regarded as a special modification, since we must consider that primitively 

 the mesoderm extended over the entire vesicle. 



The Primitive Axis. 



The next stage of development is characterized by the appearance of an 

 accumulation of cells which extends forward from the primitive knot in the axial 

 line. This thickening is termed the primitive axis. German writers commonly 

 designate it as the "head process" (Kopffortsatz). The primitive axis may be 

 easily distinguished in transverse sections from the primitive streak by the fact 

 that in the former the thickening occurs in the mesoderm and entoderm, which 

 are closely united, and it is separated from the outer layer; whereas in the latter 

 the cells of the thickening are fused with both the entoderm and the ectoderm. 



The primitive axis corresponds to the region in which the body proper of 

 the embryo develops, and represents the beginning of embryonic development in 

 this restricted sense. It grows quite rapidly in length and width, and as it grows 

 encroaches more and more upon the territory of the primitive streak, which is 

 gradually obliterated by merging into the caudal end of the developing embryo, 

 so that it can no longer be distinguished. The obliteration of the primitive streak 

 is gradual, and there is a series of stages easily observed in amniota in which 

 we find the embryonic development in the region of the primitive axis more or 

 5 



