ORIGIN OF THE MESODERM. 



63 



mole is shown in Fig. 23. At about the time the primitive streak appears tlie 

 embryonic shield becomes oval in form. In those animals, such as the carnivora 

 and ungulates, which have a large elongated blastodermic vesicle, we find that 

 the long axis of the embryonic shield is nearly at right angles to the long axis of 



Sh. 



Kn. 



t.s. 



Fig. 19. — Surface View of the Embryonic Shield of the Blastodermic Vesicle of a Dog of 

 Thirteen to Fifteen Days (Precise Age Unknown). 



The specimen had been preserved with sublimate and stained with borax-carmin. Sh, Embryonic shield. Kn, 

 Hensen's knot, p.s. Primitive streak. X 'OO diams. — {After Bonnet.) 



the vesicle. The size of the shield is about the same in all mammals which have 

 been heretofore studied. 



Origin of the Mesoderm. 



The development of the primitive streak and groove is accompanied by the 

 appearance of the third or middle germ-layer, the mesoderm (Fig. 20, mes). As 

 shown in the section there figured, the three germ-layers are fused together under- 

 neath the primitive groove, and are there thicker than elsewhere. As we pass 

 laterally from the groove, the ectoderm and mesoderm both become thinner and 

 are distinctly separated from one another. The entoderm consists of a single 



