DEVELOPMENT OF THE LEOPARD FROG 



411 



anterior end, where the brain is to form, and also posteriorly. 

 Since the neural folds extend on either side and finalh^ fuse pos- 

 teriorly in such a way as to include the anterior part of the blasto- 

 pore, a neurenteric canal (Fig. 215 E) is formed and persists for 

 a time as a communication between the neural canal and the 

 enteron. The anal opening arises as an invagination, the procto- 

 dceum, at the ventral side of the blastopore and outside the area 



m.s 



ne c 



Fig. 215. — Xeural-fold stages and transition to tadpole stage in frog. A, 

 neural-fold stage, from the left, turned slightly to show folds at anterior end 

 (r/. Fig. 216 A). B, later stage when neural folds have closed to form 

 neural tube. C and D, transition to tadpole. E, an early tadjwle stage in 



median section. 



a, anus or proctodaeum; br, brain; c, notochord; e, archenteron; ec, ectoderm; en, 

 endoderm; ep, epiphysis; ej/, eye or optic vesicle; 9. p., gill plate; A, heart; hj/, hypophysis; 

 I, liver; m.g., mucous gland or sucker; m, mesoderm; m.s., mesodermal segment; ne. c, 

 neurenteric canal; n.f., neural fold; n.p., sensory plate; sp.c., spinal cord; y.c, yolk-laden 

 cells. 



included by the neural folds in the formation of the neurenteric 

 canal. With the growth of the tail in later stages (c/. Figs. 215 E 

 and 217), the neurenteric canal extends posteriorly as a hairpin- 

 shaped tube, which gradually degenerates. These details might 

 have been omitted for the purposes of the present description, but 

 an understanding of the origin and fate of the neurenteric canal k 

 necessary' for an understanding of the more important changes in 

 this region of the embrj-o. 



