562 SEGMENTATION IN VARIOUS tYPES CHAP. 



mk'') in contact with the endoderm and with it forming the 

 splanchnopkure or wall of the enteric tube (compare p. 203). 

 Thus the body-wall and the enteric canal are separated 

 by a cavity, the calotne {D, Ih), which, much as in the adult 

 Earthworm, is divided into a series of metamerically ar- 

 ranged portions : later on, however, the adjacent walls of 

 these coelomic sacs disappear, and the coelome becomes a 

 continuous cavity. 



The embryo Amphioxus is hatched soon after reaching 

 the gastrula-stage, when it moves about by means of cilia 

 developed on the ectoderm cells and has to get its own 

 living, having by this time used up its small reserve of 

 yolk. It then passes through a complicated series of 

 larval stages, gradually leading up to the adult form. 



Early development of other types. — The presence of a 

 greater amount of food-material in the egg renders it 

 possible for the embryo to go on developing further than 

 the gastrula-stage before being hatched, and as a general 

 rule, the greater the relative quantity of yolk present in the 

 ovum of an animal, the less clearly can a gastrula-stage be 

 recognised. 



In the earthworm and mussel the segmentation is entire, but unequal, 

 and the larger, lower cells become invaginated to form the endoderm 

 and archenteron while the smaller upper cells give rise to the ectoderm. 

 In the earthworm the blastopore does not become closed, but gives 

 rise to the mouth. 



In the frog (p. 201) the archenteron arises by a. split appearing 

 amongst the yolk-cells, beginning at the edge of the blastopore 

 and gradually extending forwards : the process is probably supplemented 

 by a limited amount of invagination of the ectoderm. The archenteron 

 is at first a very narrow cleft, but soon widens considerably (Fig. 64, 

 F, ent) : for some time it does not actually communicate with the ex- 

 terior, the blastopore (blf) being filled up by a yolk-plug {yk. pT). 

 As the archenteron extends forwards, and the relatively small segment^ 



