64 



TEXT-BOOK OF EMBRYOLOGY. 



en 



nc 



b 



necessary still to consider briefly the appearance of the neural tube and some 

 further changes in the mesoderm. During the latter part of the gastrula- 

 tion period a band of ectoderm extending forward from the dorsal lip of the 

 blastopore over the dorsum of the gastrula becomes slightly thicker. This 

 band of cells, the neural plate, is narrow near the blastopore and becomes 



broader farther forward (Fig. 

 36,5). During the withdrawal 

 of the yolk plug and the closure 

 of the blastopore the margin of 

 the plate becomes thicker and 

 elevated above the surface level 

 to form the neural ridges. The 

 depression between the ridges 

 is the neural groove (Fig. 33). 

 At the cephalic end of the plate 

 the ridges curve medially and 

 meet each other, forming the 

 transverse neural fold. The 

 ridges grow higher, the groove 

 becoming correspondingly 

 deeper, and finally lean far 

 enough toward the median 

 sagittal plane to meet and fuse 

 in the mid-dorsal line, so that a 

 tube is formed with the lumen 

 as the central canal. The fusion 

 of the ridges usually begins 



ms 



ht 



FIG. 



37. Median sagittal sections of frog larvae. 

 Marshall. A , just prior to closure of blastopore ; 

 B, just after closure of blastopore. a, anal 



aperture; &, blastopore; e, epiphysis; ec, ecto- a u m]t mirlwav between their 



derm; en, entoderm; /, fore-brain; g, mid-gut; h, ' 



hind-brain; ht, rudiment of heart; hy, hypophy- cephalic and caudal ends, and 



sis; I, rudiment of liver; m, mid-brain; ms, meso- - . 



derm; n, no tocord; nc, neurenteric canal ; o, oral then continues torward and 



evagination; p proctodaeum; ph i, pharynx; r, backward. The Caudal portion 



rectum; s, spinal cord; y, yolk entoderm. 



of the neural tube encloses the 



dorsal part of the blastopore which thus, as in Amphioxus, becomes the 

 neurenteric canal, the communicating aperture between the central canal and 

 the archenteron (Fig. 3 7) . After the dorsal closure of the tube the non-neural 

 ectoderm forms a continuous layer so that the tube is completely covered 

 (Fig. 33). The broader cephalic portion of the neural tube is the rudiment 

 of the brain, the narrower remaining part is the beginning of the spinal cord. 

 In the mesoderm lateral to the neural plate and notocord the cells for 

 a time proliferate more rapidly than elsewhere and produce a rather stout 

 mass (Fig. 33) extending from the head to the blastopore. From this mass 



