282 FORMATION OF ORGANS. 



The mesoblastic masses of the head ai-e probably not to be regarded as 

 forming a pair of somites equivalent to those in the trunk, but as forming 

 the mesoblastic part of the prse-oral lobe, of which so much has been said in 

 the preceding pages. Kleiuenberg's observations are however of great im- 

 portance as shewing that the cephalic cavities are simply an anterior 

 part of the true body cavity. 



The splanchnic layer of the head cavity gives rise to the muscu- 

 lature of the O3sophagus. 



The somatic layer of the trunk somites becomes converted into the 

 musculature of the body wall and the external peritoneal layer of 

 body cavity. The first part of the muscular system to be defi- 

 nitely formed is the ventral band of longitudinal muscles which arises 

 on each side of the nervous system in contact with the epidermis (fig. 

 157, w). How the circular muscles become subsequently formed 

 outside these muscles has not been made out. 



The splanchnic layer of the trunk somites gives rise to the 

 muscular and connective-tissue wall of the mesenteron, and also to the 

 walls of the vascular trunks. The ventral vessel is first formed 

 (Kowalevsky) as a solid mass of cells which subsequently becomes 

 hollowed out. The dorsal vessel in Lumbricus and Criodrilus is 

 stated by Kowalevsky and Vejdovsky to be formed by the coales- 

 cence of two lateral vessels; a peculiarity which is probably to be 

 explained by the late extension of the mesoblast into the dorsal region. 



The layer from which the sacks for the setoe and the segmeutal 

 organs spring is still doubtful. The sacks for the setae are believed by 

 Kowalevsky (No. 342) to be epiblastic imaginations, but are stated 

 by Hatschek (No. 339) to be mesoblastic products. For the develop- 

 ment of the segmental organs the reader is referred to the chapter 

 on the excretory system. 



In marine Polycha3ta the generative organs are no doubt meso- 

 blastic products, as they usually spring from the peritoneal epithelium, 

 especially the parts of it covering the vascular trunks. 



The Alimentary Canal. Iu Lumbricus the enteric cavity is formed 

 during the gastrula stage. In Criodrilus the hypoblast has at first 

 no lumen, but this becomes very soon established. In Euaxes on the 

 other hand, where there is a true epibolic gastrula, the mesenteron is 

 at first represented by a solid mass of yolk (i.e. hypoblast) cells. As 

 the central amongst these become absorbed a cavity is formed. The 

 protoplasm of the yolk cells which line this cavity unites into a con- 

 tinuous polynuclear layer containing at intervals masses of yolk. 

 These masses become gradually absorbed, and the protoplasmic wall 

 of the mesenteron then breaks up into a cylindrical glandular epithe- 

 lium similar to that of the other types. 



In Lumbricus and Criodrilus the blastopore remains as the mouth, 

 but in Euaxes a new mouth or rather stomodseum is formed by an 

 epiblastic invagination between the front end of the two mesoblastic 

 bands. This epiblastic invagination forms the permanent oesophagus; 

 and in Lumbricus trapezoides and Criodrilus, where the oral opening is 



