592 



INVERTEBKATA 



CHAP. 



S => 



_ __ 



4 r 



I 1 



II 



the front end of the nerve plate. Here, indeed, for a considerable time 

 an aperture persists the so-called neuropore. It follows that, at the 

 hinder end of the animal, a short vertical canal, covered over by the 



.,. neural folds, leads upwards from the 

 | = archenteric cavity to the canal of the 

 1 1 nerve chord. This is the so-called 

 >.| neurenteric canal (n.e, Fig. 431, D). 

 In a later stage of development 

 the anus opens at the lower end 



I \ of this canal. The anus may be re- 



garded, consequently, as a remnant of 



I 1 the lower edge of the blastopore, and 

 indeed in the Urodela this portion of 

 the blastopore never closes, but re- 

 mains permanently open as the anus. 



As soon as the blastopore is closed, 

 the formation of the notochord and 

 of the coelom takes place. The 

 notochord originates from the mid- 

 dorsal region of the archenteron. 

 Here a strip of cells, in which the 

 yolk - granules have disappeared, 

 becomes folded up into an arch (ch, 

 Figs. 431, 432), which becomes cut 

 oft' from the rest of the endoderm 

 as a tube with a minute cavity. 

 This cavity speedily vanishes and the 

 notochord thus becomes a solid rod of 

 cells. The radial arrangement of the 

 cells in the rod soon becomes lost, i.e. 

 the cells no longer meet in a point, 

 but interdigitate with one another. 



The coelom arises as two pairs 

 of evaginations of the dorso-lateral 

 walls of the gut. The front pair 

 might be described as a pair of 

 pockets, the hinder, more correctly, 

 as a pair of longitudinal coelomic 

 grooves, the front ends of which 

 overlap the pockets. The front pairs 

 we shall compare to the collar- 

 coelomic cavities (col.coe, Fig. 431) of 

 the Enteropneusta, the hinder pair to 

 the trunk-coelomic cavities of those 

 animals. The walls of these 

 coelomic sacs constitute the mesoderm. After the neural plate is 

 covered over by the meeting of the neural folds, it becomes bent 

 into the form of a U (Fig- 432^ and finally, by the meeting of 



El 



iff 



a 



S tfj 



a:! 



, . ~ 



II 



