580 DEVELOPMENT OF LANCELET CHAP. 



the central canal of the medullary or neural cord (L, n.t) 

 is formed after the plate is separated from the outer 

 ectoderm. In the mid-dorsal line a thickening of the 

 endoderm (K-N, not) soon becomes constricted off 

 to form the notochord (pp. 203, 419, and 441), and on 

 either side of this a series of hollow endodermic 

 pouches arise from the archenteron, arranged meta- 

 merically (K-O, c.p, o.c.p, m.s). The cavities of these, 

 which subsequently give rise to the body-cavity, are 

 thus at first in free communication with the definitive 

 enteron and are known as enterocceles ; from their walls 

 the mesoderm l is derived. Subsequently the communi- 

 cations between the enteric and enterocoelic cavities 

 become closed, and the paired pouches gradually extend 

 between the ectoderm and endoderm, so as eventually 

 to meet one another both dorsally and ventrally (M), 

 their outer walls (parietal or somatic layer of the meso- 

 derm) 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) in 

 contact, below the notochord, with the endoderm and 

 with it forming the splanchnopleure or wall of the enteric 

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

 enteric canal are separated by a cavity, the ccelome, 

 which, much as in the adult earthworm, is divided into 

 a series of metamerically arranged portions : later on, 

 however, the adjacent walls of these ccelomic sacs dis- 

 appear, 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 



1 The terms epiblast, hypoblast, and mesoblast are often used to 

 designate the embryonic ectoderm, endoderm, and mesoderm 

 respectively. 



