40 COMPARATIVE ANATOMY 



in each, and they grow upwards and downwards, pushing their 

 way between the epiblast and hypoblast. Eventually the 

 somites of each pair meet above and below, and fuse in the 

 mid-dorsal and mid-ventral lines. The effect of this is that in 

 each pair of somites an inner layer of cells applied to the gut is 

 separated from an outer layer applied to the body wall. The 

 former may be called the splanchnopleur, the latter the 

 somatopleur, and the two are separated by a space lying out- 

 side the enteron, which is the ccelom or body-cavity. The 

 transverse partitions separating the successive pairs of meso- 

 blastic somites persist and form the septa of the adult worm. 

 The ccelom, then, is formed as a series of paired cavities 

 hollowed out in the mesoblast, the cavities of each pair meet- 

 ing and coalescing above and below. The inner layer of 

 mesoblast cells or splanchnopleur forms the musculature of the 

 gut and the ccelomic epithelium surrounding the gut, the outer 

 layer or somatopleur forms the musculature of the body-wall 

 and the ccelomic epithelium internal to it. 



Coincidently with these changes, important differentiations 

 make their appearance in the thickened epiblast lying outside 

 the mesoblast bands. On either side of the mid-ventral line 

 the epiblast is arranged in three or four distinct rows of cubical 

 cells, each row ending posteriorly in a rounded cell known as 

 the teloblast. At first these rows are part of the superficial 

 epiblast, but as development proceeds they sink inwards, are 

 overgrown by the adjacent epiblast, and come to lie between 

 it and the mesoblast bands. As the embryo increases in 

 length so do the rows increase by continued addition of new 

 cells budded off from the terminal teloblast. Of the three or 

 four rows on each side the innermost i.e. the rows nearest the 

 mid-ventral line give rise to the ventral nerve cord, and are 

 therefore called the neural rows, and their teloblasts the neuro- 

 blasts. The two succeeding rows on each side give rise to the 

 nephridia and the inner row of chaeta glands, hence they are 

 called nephric rows, and their teloblasts nephroblasts. The 

 fate of the outermost rows is rather doubtful, but probably they 

 give rise to the outer rows of chseta glands. 



As the mesoblastic somites are successively formed from 

 before backwards, the cells of the nephric rows become con- 

 verted into nephridia, one nephric cell giving rise to a 

 nephridium on either side of each somite. The development 



