THE ORIGIN OF THE NERVOUS SYSTEM. 



67 



observation blood-vessels are found over the entire sac. At the beginning of the 

 third week the diameter of the yolk-sac is about equal to the length of the embryo 

 (Fig. 25). By the end of the third week the sac has become distinctly pear- 

 shaped, its narrower pointed end being that by which it is connected with the 

 intestinal canal of the embryo (Figs. 33, 34). The sac continues growing, up to 

 the end of the fourth week, after which it enlarges very slightly, if at all. Its 

 diameter is only from 7 to n mm. It is then a pear-shaped vesicle attached by 

 a long stalk to the intestine, the stalk having been formed by the lengthening of 

 the neck of the yolk-sac. The cavity of the stalk early becomes obliterated and 

 theentoderm in the stalk disappears altogether. 



FIG. 34. HUMAN EMBRYO OF 2.6 MM. (.4/ter W. His.) 



The Origin of the Nervous System. 



It will be remembered that the ectoderm of the embryonic shield has at first 

 a considerable thickness, for it consists of cuboidal or low cylindrical epithelial 

 cells. The stage which follows next after the appearance of the primitive axis is 

 characterized by the gradual thinning out of the ectoderm over the peripheral por- 

 tions of the shield, while in the neighborhood of the axial line the full thickness of 

 the outer germ-layer is not only retained, but is actually increased. For a time 

 there is a gradual passage between thicker and thinner parts, but as development 

 progresses the demarcation rapidly becomes sharper. By these steps the differentia- 

 tion of the anlage of the central nervous system is accomplished. The thicker cen- 

 tral portion of the ectoderm constitutes the medullary plate, which begins to appear 

 shortly after the formation of the primitive streak.. It extends over the primitive 

 axis, the primitive knot, and the anterior end of the primitive streak (Fig. 35, Md), 

 and also extends some distance to the right and left of the axial line. It is rounded 



