xii SEGMENTATION IN VARIOUS TYPES 579 



from their walls the mesoderm is derived. Subsequently 

 the communications between the enteric and enteroccelic 

 cavities become closed, and the paired pouches gradually 

 extend between the ectoderm and endoderm, both dorsally 

 and ventrally (Fig. 153, c, D), their outer walls (parietal or 

 somatic layer of the mesoderm, mJP) being in contact with 

 the ectoderm and forming with it the somatopleure or body- 

 wall, and their inner walls (visceral or splanchnic layer of 

 the mesoderm, ;;// 2 ) in contact with the endoderm and 

 with it forming the splanchnopleure or wall of the enteric 

 tube (compare p. 203). Thus the body-wall and the enteric 

 canal are separated by a cavity, the ccelome (D, lh}> which, 

 much as in the adult earthworm, is divided into a series 

 of metamerically arranged portions : later on, however, the 

 adjacent walls of these coelomic sacs disappear, and the 

 ccelome becomes a continuous cavity. 



The embryo lancelet 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. 



P P 2 



