MESOTHELTAL STRUCTURES. 



103 



cells are not wholly used up in building the solid skeleton, but 

 that some wander in between splanchnic mesothelium and 

 entoderm, where they give rise to the 

 smooth muscles and connective tissue 

 of the alimentary canal, some pass be- 

 tween the myotomes, v/here they form 

 the partitions (myocommata ^ ) be- 

 tween these structures, while others 

 press farther and give rise, in part, to 

 the deeper layers (cutis) of the skin, 

 etc. This same rhiddle zone also 

 gives rise to a part of the excretory 

 system (nephridia), which is also pri- 

 marily divided into segments (ne- 

 phrotomes). 



The lateral plate region (hypo- 

 mere) shows but slight traces of 

 segmentation. From the dorsal por- 

 tion of its splanchnic wall arises the 

 glomus of the pronephros (see below) 

 and the gonads (reproductive struc- 

 tures), but whether or not these lat- 

 ter are truly segmented, and whether 

 we have metamerically repeated go- 

 notomes, is as yet a disputed question. 

 The account of these reproductive 

 and excretory organs will be given 

 later. 



Mesenteries. — The greater por- 

 tion of the lateral plates develops 

 into the flattened epithelium (peri- 

 toneum) lining the body cavity 

 (splanchnocoele or metacoele), and 

 plays an important part in the devel- 

 opment of the walls of the alimentary 

 tract and the membranous supports 



Fig. 112. Transverse sec- 

 tion of embryo dogfish (^Acan- 

 ihias'). (2, aorta; c^ ccelom; e^ 

 ectoderm ; g, ganglion of spinal 

 nerve; h, hypochorda; i, intes- 

 tine ; m, mesomere ; mc, mesen- 

 chyme ; ms, mesentery ; my, 

 myotome; «, notochord ; p, 

 pronephric duct ; s, sclerotome ; 

 sc, spinal cord ; so and sp, 

 somatic and splanchnic layers; 

 sn, spinal nerve ; w, wall of in- 

 testine. The section passes on 

 the left side through the middle 

 of a myotome, on the right near 

 the edge of one. 



t The term myocomma is sometimes regarded as a synonym of myotome ; the usage 

 adopted here is preferable ; myoseptum is another term for it. 



