44 



EVOLUTION AND GENETICS 



body becomes depressed, and flanked by two ridges (Fig. 23). 

 This process of involution finally results in a meeting and fusion 

 of the two ridges, now become folds, so that an inner longitudinal 

 tube of ectoderm is formed, surmounted by a continuous layer of 



LV PH 



^T 



Fig. 24.^Longit,udinal sections of developing embryo of frog. A, earlier, B, 

 later stage. N, notochord; Fb, Mb, and Hb, fore-, mid-, and hind-brain; 

 PT, pituitary anlage; PD, proctodaeum; A, anus; PH, enteron; NT, neural 

 tube at the neurenteric canal. (From Holmes.) 



superficial ectoderm. As a result of the relation of the folds to the 

 blastopore, the cavity of the tube communicates with that of the 

 archenteron (Fig. 24). The tube is destined to become the central 

 nervous system, and is called the neural tube, hence this con- 

 tinuous passage is called the neurenteric canal. The connection is 

 later broken, the dorsal tube persisting throughout life in the 



