IV 



VERMES— ONTOGENY OF THE WORMS 



281 



segment as outgrowths of the nephridial cell rows. Each duct consists at first of one 

 cell or of a few cells, and later of a row of cells bent in the shape of the letter U, 

 which projects into the body cavity and is thus provided with an outer endothelium. 

 One limb of the cell row remains in contact with the integument, the end of the 

 other attaches itself to the posterior wall of the dissepiment which lies in front of it. 

 At this point an inner canal first appears in the solid row of cells. The terminal 

 portion arises by an invagination of the integument. The funnel begins to form 

 from one cell on the anterior wall of the dissepiment at the point where, on the 

 posterior wall, the end of the nephridial duct lies. This cell only secondarily 

 becomes a hollow ciliated funnel, which then unites with the nephridial canal 

 through the dissepiment. The funnel thus arises separately from the nephridial 

 canal out of the epithelium of the body cavity, and not out of the nephridial 

 rows of cells. In the Folychceta also the funnel and the nephridial duct of each 

 nephridium are said to arise separately. 



Development of the Sexual Glands. — It may be considered certain that in the 

 Annulata and Prosopycjia the ovaries and testes are developed from special parts of 

 the endothelium of the body cavity. 



The Development of the Mesoderm in the Chsetognatha (Fig. 190).— The nervous 

 system here lies in the integument of the body and does not belong to the mesoderm. 



Fig. 190.— .i, -B, C, Three early stages of development of Sagltta (after 0. Hertwig). a, Gas- 

 trula ; 6Z, blastopore ; ud, arch-enteron ; (/, primitive cells of the sexual organs ; vm, visceral layer ; 

 %tm, parietal layer of the mesoderm ; d, rudiment of mid-gut ; cs, coelome sacs ; st, stomodseum ; d, 

 intestine. 



In this case the mesoderm develops in a manner different from that in the worms as 

 yet described. A ccelogastrula forms whose principal axis answers pretty accur- 

 ately to the longitudinal axis of the adult Sagitta. The aboral pole of the gastrnla 

 corresponds with the future anterior end of the body. Two large cells which soon 

 divide enter the base of the archenteron from the endoderm at an early stage. 

 These 4 cells are the rudiment of the testes and ovaries. Then on each side 

 there arises out of the base of the archenteron a fold of the endoderm, which grows 

 into the archenteric cavity towards the blastopore. These 2 folds divide the arch- 

 enteric cavity into a central cavity and two lateral cavities, which communicate at 

 the free edges of the folds. The central cavity is the definitive enteric cavity ; its 

 epithelial walls, i.e. the inner epithelial lamellse of the folds, represent the rudiment 

 of the definitive enteric epithelium. The folds close dorsally and ventrally to form 

 the enteric tube, the latter carrying the 4 sexual cells at its freely projecting end. 

 The two lateral cavities, which may almost be looked upon as 2 sac-like invaginations 

 of the archenteron (coelomic sacs), form the commencement of the body cavity. 

 Each sac has an outer epithelial wall in contact with the ectoderm, and an inner 

 wall in contact with the enteric tube. The former is the parietal, the later the 

 visceral layer of the mesoderm. The former probably forms the musculature and 

 the endothelium of the body wall, the latter the enteric endothelium. At the aboral 



