502 SEGMENTATION IN \'ARIOUS TVl'ES ciiAP. 



iiili') in contact with the endoderm and witii 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 cavit}-, the iwloine {D, Ih), which, much as in the adult 

 Earthworm, is divided into a series of nietamerically ar- 

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

 these ccelomic sacs disappear, and the c( elonie 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 gel its own 

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

 )olk. 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 amouait 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 quantit}' of yolk present in the 

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

 recognised. 



In llic uarlliwunii and nuis.scl Ihc segmciiUilitini i>cntir(.', bul unequal, 

 und Ihc kirgcr, low tr cells becumc invaginalcd to foini llic cnclodcrni 

 and archcnleion while the smaller upper celKyive rise to the eeludenii. 

 In the earLhworni the blaslopore does nol l.tecetnie closed, Iiul !^i\es 

 rise lo the mouth. 



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

 amongst the yolk cells, beginning at the edge of Ihc blastopoie 

 andgradually extending forwards : the j'locess is inobably su|)plcnicnlcil 

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

 is at first a very narrow cleft, but soon widens consider. dily (Eig. 64, 

 F,cnl): for some time it docs not actually communicate with the e\- 

 tcrloi, the blastopore (/>/p) being filled up by .t yolk-plug ( y/,\ pi). 

 As the archenteron extends forwards, and the iclati\ely small segment 



