172 



GENERAL BIOLOGY 



inner one of which (the splanchnic layer) becomes applied 

 against the endoderm to form the larger part of the wall of 

 the enteron; the other (the somatic layer), applied 

 to the ectoderm, becomes much the greater part of the body 

 wall. The cleft between these layers is the ccelom. The 

 blastopore in the worm becomes the mouth ; the breaking 

 through of the tissues at the opposite end of the body 

 transforms the primitive food sac into an alimentary 

 canal having the obvious advantage of permitting unin- 

 terrupted passage of food, and facilitating also the struc- 

 tural and physiological differentiation of parts along the way. 



Thus at a very early stage of its development, the fun- 

 damentals of the plan of structure of the earthworm are 

 clearly established. 



Later development. After the completion of the enter- 

 on, there occurs along with the rapid elongation of the 

 body, an ingrowth of the ectoderm at both ends (but 

 principally at the front end) which results in the restriction 

 of the endoderm to that part already designated in the 

 adult worm as stomach-intestine. The diagram of figure 

 103^ indicates roughly the distribution the three germ 

 layers acquire. 



The three are unlike in the nature and extent of the 

 differentiation of their cells in the formation of tissues. The 

 embryonic endoderm becomes the adult epithelium diges- 

 tive epithelium (and that only) in the worm. The ectoderm 

 differentiates chiefly into two sorts of cells: i) into epider- 

 mis, which remains in the original position on the surface 

 of the body and fulfills the primitive function of protection ; 

 and 2) nerve cells, which are separated off from the ectoderm 

 upon the ventral side as indicated in figure io8c, d, e, and 

 pass between the developing masses of mesoderm to lie 

 within the ccelom and to develop there the nervous tissue 

 (covered, however, by an investment that is of mesodermal 

 origin) . 



