THE PHYLUM CHORDATA 



47 



the gastrula emerges from the vitelline membrane as a free-smmming 

 larva. It is now an elongated gastrula and has begun to develop the 

 rudiments of definitive 

 structures, such as noto- 

 chord, ccelom, and med- 

 ullary plate. The first 

 signs of metamerism are 

 seen in connection with 

 a series of paired lateral 

 pouches derived from 

 the archenteron, (Fig. 

 19 A) beginning near the 

 anterior end and pro- 

 ceeding posteriorly un- 

 til fourteen pairs are 

 formed. Additional seg- 

 ments arise as a direct 

 outgrowth of the hind 

 end of the body. 



The medullary plate 

 (Fig. 18) is cut off from 

 the ectoderm of the 

 body wall in a pecuhar 

 way by the cooperation 

 of two factors. The 

 ectodermal ridges at the 

 side of the plate arch 

 over the middle parts of 

 the plate and the ecto- 

 derm of the ventral lip 

 of the blastopore grows 

 like a sheet over the 

 blastopore and closes 

 the latter so that instead 

 of opening to the outside 

 it communicates with 



Fig. 20. — Sections through young Amphioxus 

 embryos showing the origin of the anterior gut 

 diverticula. A. Frontal section through embryo 

 with nine pairs of somites. (See Fig. 19, B). The 

 dotted line marks the course of the gut wall ven- 

 tral to the level of the section. B. Optical sagit- 

 tal section through anterior end of embryo with 

 thirteen pairs of somites showing position of right 

 anterior gut diverticulum. C. Same in ventral 

 view, c, ccelomic cavity of somite; ch, notoohord; 

 csg, rudiment of club-shaped gland; d, rudiment of 

 anterior gut diverticula; ec, ectoderm; en, endo- 

 derm; g, gut cavity; gsl, rudiment of first giU slit; 

 Id, left anterior gut diverticulum; n, nerve cord; 

 np, neuropore; rd, right anterior gut diverticulum; 

 si, S2, S3, first, second, ninth mesodermal somites. 

 (From Kellicott's " Outline of Chordate Develop- 

 ment " [Henry Holt and Co.] after Hatschek.) 



the neural tube and has become a neurenteric canal, the homologue of 

 which is found in all vertebrates and constitutes one of the most 

 peculiar characters of the group. 



