126 



EMBRYOLOGY 



45 



36 



4Q 



50 



17 58 



Jm 



BEGINNING NEURAL 



PLATE 



NEURAL PLATE 



BLASTOPORE 



NEURAL PLATE FOLDING 

 NEURAL FOLD 



BLASTOPORE 



®: 



NEURAL PLATE OPEN 

 ANTERIORLY 



CLOSED NEURAL TUBE 



NEURAL TUBE COMPLETELY 

 CLOSED 



GILL PLATE 



Fig. 67. From early neural plate to tail 

 bud in Rana sylvatica. The most striking 

 changes in a study of the external appear- 

 ances occur in the neural plate. The flat plate 

 of stage 13 curls in stage 14, and the edges 

 of the plate meet (stage 15) to form a closed 

 tube (stage 16). The epidermis fuses over 

 the neural tube (stage 17). (From Pollister 

 and Moore, Anatomical Record (1937), 68, 

 489.) 



Segregation of the notochord and 

 mesoderm from the endoderm 



While the neural plate is converted into a neural tube, the lateral meso- 

 derm and notochord become segregated from the endoderm. The formation 

 of the lateral mesoderm can best be illustrated in cross sections of the late 

 gastrula and early neurula. Let us first, however, examine the relationship of 



