mi; . if 'iin: p 117 



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

 to tin- adjacent epideii; 



In all tlu> claasee of Vertebrates tbe medullary plat.- i- trm. 



into a m ' at a very early period. TliU pnv>- can be 



atcompli>hed in (hive dill'erent \\ay>. In most of the classes of 

 Yertehr.-iU's. namely lleptilrs. Ilirds. and Mammal-, the tui 

 formed by a typical process of folding. Tin- medullary folds i i- 

 .-till higher aU)\e the Mirfaee of the ^crm. t lien li-nd tcgHhi-i 

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

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

 thu> formed, >till continues to remain in connection with the over- 

 lying epidermi> along the line of fusion, a connection which soon 

 disappears, since the connecting eells become loosened and separated 

 from one another (tig. 41 0). The closure begins in all Vertebr 

 at the place which corre>pond> approximately to the future mid-brain 

 in the Chick (fig. 87 hb~) on the second and in the Kabbit on the 

 ninth day of development and from there proceed^ -lowly 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 

 iieurenteric canal al>o c\ist> at the posterior end, a> has been already 

 mentioned (p. 12G) in the discussion of the ^erm-layeis. It is only 

 at a later period that this connection is interrupted by the cWii 

 the blastopore. 



The second (;// in the development of the central nervous system 

 is met with in Cyclostoines and Teleosts. In them the neural plat, 

 is transformed into a solid cord of cells instead of a tube. 3 1 

 the folds rising up over the Mil-face of the germ, the neural p 

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

 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 

 bet ween them ; only after the cord of cell> ha> 1-e n constricted <1Y 

 from the primitive epidermis do the hakes separate and allow ;t 

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

 this modification in the Bony Ki>he> and ('vcloM<>in.'- i- connected 

 with the fact that the tgir with its abundant yolk is very closeh 

 enveloped by the vitelline meml.rane, a- a i.-ult of which tin- 

 medullary folds cannot rise toward the >urfa<.'. 



'Eke third iu> > occurs only in Amphioxu> lain eolatus. It 



lias already been docribed briefly in anoth. r place (p. 109). 



The neural tube r tains an unditVerentiated condition in Amphioxus 



27 



