THE ORIGIN OF THE NERVOUS SYSTEM. 



69 



A.o 



md.F 



third and fourth month. The cavity in which these notochordal remnants are 

 lodged is supposed not to be identical with the intervertebral cavity of the adult. 



The Origin of the Nervous System. 



The first step in the differentiation of the central nervous system is the 

 formation of a thickening of the ectoderm, which is known as the medullary 

 plate, and 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. 

 25, Md), and also extends some 

 distance to the right and left of 

 the axial line. It is rounded in 

 front and behind, where, how- 

 ever, it gradually fades out. It 

 will be remembered that the 

 ectoderm of the embryonic shield 

 has at first a considerable thick- 

 ness, for it consists of cuboidal 

 or low cylindrical epithelial cells. 

 The stage which follows next 



pr.s 



Md 



FIG. 24. DEGENERATING TISSUE OF THE 

 NOTOCHORD FROM THE CENTRAL POR- 

 TION OF THE INTERVERTEBRAL Disc 

 OF A Cow's EMBRYO. (After Leboucq.} 



FIG. 25. SURFACE VIEW OF THE EMBRYONIC 

 SHIELD OF A DOG EMBRYO, WITH MEDUL- 

 LARY PLATE. 



A.o, Area opaca. A.p, Area pellucida. Kn, Hen- 

 sen's knot. Md, Medullary plate. md.F, 

 Medullary furrow, pr.s, Primitive streak. X 

 15 diams. 



after the appearance of the primitive axis is characterized by the gradual thin- 

 ning out of the ectoderm over the peripheral portions 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 demarca- 

 tion rapidly becomes sharper. At the same time that the medullary plate is 

 being thus differentiated, the central portion becomes depressed, making the 



