XVII 



PKOTOCHOKDATA 



593 



the edges of this, into a tube which is the neural tube <>r spinal 

 cord. During the larval life of the animal this tube opens in front 

 by the neuropore (n.p.o, Fig. 436, B). 



Soon after the formation of mesoderm an extremely rapid growth 

 in length of the embryo sets in, then hatching takes place, and the 



eft 





trcoe 



FIG. 431. Transverse sections through an embryo of Amphioxus lanceulatva just after 

 the completion of gastrulation, in order to illustrate the development of nerve cord, 

 notochord, and coelorn. 



A, section through the anterior region of the embryo, showing- the neural plate exposed and the 

 evaginations of the gut, which give rise to collar and trunk coelomic cavities. B, si'ctiuii through a 

 more posterior region of the embryo, showing the neural folds nearly meeting above the neural plate, 

 and the arching of the miil-dcirsal section of the gut to form the notochord. C, section through the 

 hinder end of the embryo, showing the anal diverticuluin and tin- union of the neural folds. I), sect ion 

 behind the last, showing the neurriiti-ric canal. .</, anal diveiticulum ; <-li . rudiment of I he notorhord ; 

 col.coe, rudiment of the collar coelom ; K.I-, nem enteric canal ; ./, neural folds; .//, neural plate ; tr.coe, 

 rudiment of trunk coelDin. 



minute larva swims around by the aid of the cilia which clothe the 

 whole ectoderm. The growth in length is chiefly localized in the 

 hinder region of the embryo, and includes the hinder ends of the 

 trunk-cavities, so that these grow iii length pari passu with the rest 

 of the larva. There is, however, a general growth in length of all 

 VOL. i 2 Q 



