PHYLUM NEMATHELMINTHES 



309 



which have become arranged in two rows. The epithelium of the 

 pha,rynx is formed by an involution of the ectoderm, so that this 

 division of the enteric canal forms a stomodcmim (Fig. 248, 1, stdm.), 

 and the rectum appears also to be lined by ectodermal cells and 

 thus to be of the nature of a proctodoeuvi. The middle layer is 

 formed from a pair of cells (D, mes.) of the inner layer which 

 enlarge and multiply to form a layer of cells between the 



Fig. '24S. — Development of Ascaris nigrovenosa. hip. blastopore ; ect. ectoderm ; end. endo- 

 derm ; ent. enterou ; g. genital cell ; nies. mesoderm ; n. nervous system ; stdm. stomodfeum. 

 (From Korschelt and Heider, after Goette.) 



ectoderm and the endoderm (E — H.) The body-cavity is not a 

 true ccfilome {i.e., a cavity formed in the interior of a mass of 

 mesoderm or arising as an outgrowth from the archenteron), but is 

 derived from the primitive blastula- or segmentation-cavity. The 

 nervous sytem is developed from certain cells (G — I, n.) which 

 bud off from the ectoderm at the anterior end of the body. The 

 reproductive cells originate from a pair of the inner cells (H, I, g.) 

 which become differentiated from the rest at an early stage. 



