GEPHYREA. 359 



During the above changes important advances are made in the forma- 

 tion of the organs from the embryonic layers. 



The epiblast acquires a superficial cuticula, which is perhaps directly 

 derived from the vitelline membrane. The nervous system is also formed, 

 probably from the epiblast. The band-like supra-cesophageal ganglion is 

 the first part of the nervous system formed, and appears to be undoubtedly 

 derived from the epiblast. The ventral cord arises somewhat later, but the 

 first stages in its development have not been satisfactorily traced. It is 

 continuous with the supra-cesophageal band which completely girths the 

 oesophagus without exhibiting any special dorsal enlargement. After the 

 ventral cord has become completely separated from the epiblast a central 

 fibrous mass becomes differentiated in it, while the lateral parts are composed 

 of ganglion cells. In the arrangement of its cells it presents indications of 

 being composed of two lateral halves. It is, however, without ganglionic 

 swellings. 



The mesoblast, though at first very thin, soon exhibits a differentiation 

 into a splanchnic and somatic layer though the two do not become 

 distinctly separated by a body cavity. The somatic layer rapidly becomes 

 thicker, and enlarges laterally to form two bands united dorsally and 

 ventrally by narrow, thinner bands. The outermost parts of each of these 

 bands become differentiated into an external circular and an internal 

 longitudinal layer of muscles. In the pras-oral lobe the mesoblast assumes 

 a somewhat vacuolated character. 



The hypoblast cells form a complete layer round the four yolk cells from 

 which they arise (fig. 162 B, al\ but at first no alimentary lumen is developed. 

 The oesophagus appears during this period as an, at first solid, but subse- 

 quently hollow, outgrowth of the hypoblast towards the epiblast. 



The metamorphosis of the larva into the adult female 

 Bonellia commences with the conversion of many of the in- 

 different mesoblast cells into blood corpuscles, and the intro- 

 duction into the body cavity of a large amount of fluid, which 

 separates the splanchnic and somatic layers of mesoblast. The 

 fluid is believed by Spengel to be sea-water, introduced by two 

 anal pouches, the development of which is described below. 



The body cavity is lined by a peritoneum, and very soon 

 distinct vessels, formed by folds of the peritoneum, become 

 established. Of these there are three trunks, two lateral and a 

 median in the prae-oral lobe (proboscis), and in the body a 

 ventral trunk above the nerve cord, and an intestinal trunk 

 opening anteriorly into the ventral one. The vessels appear 

 to communicate with the body cavity. 



In the course of the above changes the two ciliated bands 



