THE ORGANS OF THE OUTER GERM-LAYER. 417 



the marginal part of the neural plate, the outer, on the contrary, 

 to the adjacent epidermis. 



In all the classes of Vertebrates the medullary plate is transformed' 

 into a neural tube at a very early period. This process can be 

 accomplished in three different ways. In most of the classes of 

 Vertebrates, namely Eeptiles, Birds, and Mammals, the tube i* 

 formed by a typical process of folding. The medullary folds rise 

 still higher above the surface of the germ, then bend together 

 toward the median plane, and grow toward each other until their 

 edges meet, along which they then begin to fuse. The neural tube, 

 thus formed, still continues to remain in connection with the over- 

 lying epidermis along the line of fusion, a connection which soon 

 disappears, since the connecting cells become loosened and separated 

 from one another (fig. 41 C). The closure begins in all Vertebrates 

 at the place which corresponds approximately to the future mid-brain 

 in the Chick (fig. 87 hb 2 ) on the second and in the Rabbit on the 

 ninth day of development and from there proceeds slowly both 

 backwards and forwards. There is retained for a long time, 

 especially behind, a place where the neural tube is open to the- 

 exterior. A connection with the intestinal tube by means of the 

 neurenteric canal also exists at the posterior end, as has been already 

 mentioned (p. 126) in the discussion of the germ-layers. It is only 

 at a later period that this connection is interrupted by the closing of 

 the blastopore. 



The second type in the development of the central nervous system 

 is met with in Cyclostomes and Teleosts. In them the neural plate 

 is transformed into a solid cord of cells instead of a tube. Instead of 

 the folds rising up over the surface of the germ, the neural plate 

 grows downward in the form of a wedge. In this way the right 

 and left halves of the plate come to lie immediately in contact with 

 each other, so that one cannot find the slightest trace of a space 

 between them ; only after the cord of cells has been constricted off 

 from the primitive epidermis do the halves separate and allow a 

 small cavity, the central canal, to appear between them. Probably 

 this modification in the Bony Fishes and Cyclostomes is connected 

 with the fact that the egg with its abundant yolk is very closely 

 enveloped by the vitelline membrane, as a result of which the 

 medullary folds cannot rise toward the surface. 



The third modification occurs only in Amphioxus lanceolatus. It 

 has already been described briefly in another place (p. 109). 



The neural tube retains an undifferentiated condition in Amphioxus- 



27 



