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PKOTOCHOKDATA 



593 



the edges of this, into a tube which is the neural tube or 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 



trcoe 



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

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

 notochord, and coelom. 



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, section through a 

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

 and the arching of the mid-dorsal section of the gut to form the notochord. C, section through the 

 hinder end of the embryo, showing the anal diverticulum and the union of the neural folds. D, section 

 behind the last, showing the neurenteric canal, a.d, anal diverticulum ; ch, rudiment of the notochord ; 

 col.coe, rudiment of the collar coelom ; n.e, neurenteric canal ; n.f, neural folds ; n.p, neural plate ; tr.coe, 

 rudiment of trunk coelom. 



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 in length pari passu with the rest 

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



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