100 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



MESOTHELIAL STRUCTURES. 



THE mesothelial structures, as we left them on a preceding 

 page (p. 8), consisted of a pair of compressed sacs or pouches, 

 r one on either side of 



the entodermal tube. 

 Each pouch consists of 

 an inner or visceral, 

 and an outer or soma- 

 tic, wall, the cavity be- 

 tween them being the 

 primitive coelom, which 

 is now entirely cut off 

 from all other cavities. 

 The derivatives of this 

 mesothelial tissue are 

 to be described, 



FIG. 109. Transverse section of Amblystoma 

 embryo after the separation of the mesothelium. 

 a, archenteron ; c, coelom ; m, mesothelial walls of 

 coelom ; n, notochord ; r, groove of closure of 

 neural tube ; s, spinal cord ; /, canal of spinal cord ; 

 y, yolk. 



now 



but it must be kept 

 in mind that there are 

 many exceptions to the details as given below. The state- 

 ments regarding the somites apply most nearly to the elasmo- 

 branchs, but they are generalized in many respects. 



First in order is the development of the primitive segments 

 or somites of the body. It is to be noted that while other parts 

 are segmentally or metamerically arranged (nerves, blood-ves- 

 sels, skeleton), this metamerism primarily arises in the mesothe- 

 lium, and becomes secondarily impressed upon other structures. 

 The process of somite formation is best seen in the trunk 

 region. 



As a result, partly of the change in the shape of the em- 

 bryo caused by the infolding of the medullary plate (see ner- 

 vous system), in part of the growth of the mesothelium itself, 

 the ccelomic pouches extend upwards from their primitive posi- 



