118 OUTLINES OF CHORDATE DEVELOPMENT 



brain. This is, therefore, the last region of the neural tube to 

 close, and may consequently be termed the neuropore; in the 

 later embryo this is the region just posterior to the epiphysis or 

 pineal body (see next chapter). The neuropore has a very 

 transitory existence. 



en 



nc 



b 



B 



ms 



ht 



FIG. 37. Diagrams of median sagittal sections of frog embryos. After 

 Marshall. A. Just before the closure of the blastopore. B. Just after the 

 closure of the blastopore. (See Fig. 38, D, E.~) a, Anal or cloacal aperture; 

 b, blastopore; e, epiphysis; ec, ectoderm; en, endoderm;/, fore-brain; g, mid-gut; 

 h, hind-brain; ht, rudiment of heart; hy, hypophysis; I, liver diverticulum ; m, mid- 

 brain; ms, mesoderm; w, notochord; nc, neurenteric canal; o, oral evagination; 

 p, proctodseum; ph, pharyngeal region of gut cavity; r, rectum; s, spinal cord; 

 y, yolk cells. 



Those cells in the nervous layer of the ectoderm forming the 

 lateral margins of the neural plate, that is, the neural ridges 

 proper, do not themselves form an integral part of the neural 

 tube. When the margins of the neural plate fold together they 

 are left dorso-laterally, between the neural tube and the defini- 

 tive ectoderm. These ridges of cells become broken into cell 

 groups, lying along the lateral regions of the neural tube, forming 



