468 



DEVELOPMENT OF PRIMITIVE BODY FORM 



RHOMBENCEPHALON 

 MESENCEPHALON 

 PROSENCEPHALON 



NEURAL TUBE 



NOTOCHORD 



MESENCHYME 



AREA 



ORAL EVAGINATION 



ORAL SUCKER 



NEURAL TUBE 

 HEAD GUT 



SUBNOTOCHOROAL 

 ROD 



VENTRAL MESODERM 

 NOTOCHORD 



FOREGUT 



MIDGUT 



Fig. 225. Structure of 3'/2- to 4-mm. embryo of Rana pipiens (about eight pairs of 

 somites are present). (See fig. 226A and B for comparable external views of lateral and 

 ventral aspects of 5-mm., Rana sylvatica embryo.) (A) External dorsal view. (B) Mid- 

 sagittal view. (C) Same, showing major organ-forming areas. 



scribed as a neurula, especially in the Amphibia. However, in the bird and 

 the mammal, the embryo during this period is described in terms of the number 

 of somitic pairs present, and this stage in these embryos is referred to as the 

 somite stage.) Each lateral neural fold continues to move dorsad and mesad 

 until it meets the corresponding fold from the other side. When the two neural 

 folds meet, they fuse to form the hollow neural tube and also complete the 

 middorsal area of the epidermal tube (cf. figs. 221, 224, 233, 234, 236, 237, 

 242, 245A). As a general rule, the two neural folds begin to fuse in the 

 anterior trunk and caudal hindbrain area. The fusion spreads anteriad and 

 posteriad from this point (figs. 223, 229, 233, 235, 242, 245A). It is im- 

 portant to observe that there are two aspects to the middorsal fusion process: 



(a) The lateral edges of the neural plate fuse to form the neural tube; and 



( b ) the epidermal layer from ehher side fuses to complete the epidermal 

 layer above the newly formed neural tube. 



Associated with the fusion phenomena of the epidermis and of the neural 

 tube, neural crest cells are given oflf or segregated on either side of the neural 

 tube at the point where the neural tube ectoderm separates from the skin 



