CHAP, xni DEVELOPMENT OF LANCELET 579 



bp. blastopore ; blc. segmentation-cavity ; cp. coelomic pouch (enterocoele) ; ent. 

 enteron ; ep. ectoderm ; hyp. endoderm ; m.s. mesodermic segments ; not. noto- 

 chord ; n.p. neuropore, or anterior opening of medullary tube ; n.pl. medullary 

 plate ; n.t. medullary tube ; o.c.p. openings of ccelomic pouches into enteron. 

 (From Dendy, adapted from Ziegler's models, after Hatschek.) 



its cavity is the primitive enteron or archenteron (ent), 

 and is bounded by the invaginated cells which now 

 constitute the endoderm (hyp), the remaining cells form- 

 ing the outer wall of the gastrula being the ectoderm (ep} 

 (compare pp. 201 and 202). The two layers are con- 

 tinuous at the aperture of the cup, the gastrula-mouth or 

 blastopore (bp) . Between the ectoderm and endoderm is 

 at first a space, the greatly diminished segmentation- 

 cavity, which soon becomes entirely obliterated, so that 

 the ectoderm and endoderm are in contact. The 

 general resemblance of the gastrula to a simplified 

 Hydra, 1 devoid of tentacles, will be at once apparent, 

 and the stage in the development of the frog's egg 

 represented in Fig. 64, I, though much modified by the 

 quantity of food-yolk (compare p. 574), will be seen to 

 correspond to the gastrula-stage. As in the frog, the 

 blastopore soon closes over, the mouth and anus being 

 subsequently formed from the stomodaeum and the 

 proctodaeum respectively (p. 204). 



The gastrula becomes elongated, flattened on one 

 side and convex on the other (J). The flattened side 

 corresponds to the dorsal surface of the adult, and the 

 blastopore now comes to be situated, as in the frog- 

 embryo (Fig. 64, H, K), at the posterior end of the dorsal 

 surface. A medullary or neural plate and groove (K, n.pl) 

 are then formed, the central nervous system being de- 

 veloped in a manner essentially similar to that already 

 described in the case of the tadpole (p. 202), except that 



1 It must, however, be remembered (pp. 308 and 321) that the 

 ectoderm and endoderm of Hydroids are differentiated before the 

 mouth is formed, so that the mouth does not correspond to the 

 blastopore of the gastrula. 



