XVII 



PEOTOCHOKDATA 



593 



the edges of this, iuto 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 talces place, and the 



trcoe 



Pio. 431. — Transverse sections throngh an embryo of Amphioxus lanceolatiis just after 

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

 notochord, and ooelom. 



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

 evaginations of the giit, 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. 0, 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 ; c/i, rudiment of the notochord ; 

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

 rudiment of trunk coelom. 



miijute larva swims around by the aid of the ciHa which clothe the 

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

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

 truflk-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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