NOTES ON EMBRYOLOGY AND CLASSIFICATION. 431 



cell- elements which mal^e up nerve tissue may be evenly 

 distributed along these tracts, or the spherical elements may 

 be concentrated at important points in obedience to the pro- 

 portions assumed by other parts of the body, e. g. metameric 

 segmentation of muscles, or development of a mesopod 

 (moUuscan foot), or of a special sense organ. The lateral 

 tracts once established show the strongest tendency to unite 

 into a single tract by gradually taking up a median position. 

 This junction of the lateral tracts may occur either dorsally 

 orventrally, and the result is the production of a dorsal or of 

 a ventral nerve mass. In most of the higher Enterozoa the 

 junction is effected dorsally in the region of the prostomium 

 and ventrally in the metastomial region. Cases of very partial 

 or altogether ineffective fusion of the metastomial portions 

 of the nerve tracts are common. 



Archenteron, Paretitera, Metenteron, Mesenteron, and 

 Hepatic Caeca — The successive differentiations or subdivi- 

 sions of the original digestive cavity (archenteron), lined by 

 the endoderm or enteric cell layer, may be rapidly summa- 

 rised as follows: — The archenteron (Urdarm) breaks up into 

 the two (subsequently fused) parentera and the axially- 

 placed metenteron. The parentera become coelom, the meten- 

 teron retains digestive functions. The parentera form coelomic 

 and vascular epithelium, blood-corpuscles and female repro- 

 ductive tissue (ova). According to Ed. Van Beneden the 

 male reproductive tissue is formed from ectodermal (deric) 

 cells. The metenteron is joined by stomodseum and procto- 

 dseum, and now gives rise in a large number of cases to two 

 csecal outgrowths (the hepatic cseca), often of great size, which 

 resemble in some cases the coelomic parentera. According to 

 the view here taken, however, they have nothing to do with 

 the coelomic parentera, but are of much later origin. They 

 become widely separated in character from the rest of the 

 metenteron, which must now be distinguished as mesenteron, 

 and continue to open into it by a narrow passage, through 

 which their secretion passes. Thus, then, as archenteron 

 divides into parentera and metenteron, so metenteron divides 

 into hepatentera or hepatic cseca and mesenteron. 



Other diverticula to which the mesenteron give^ rise do 

 not require notice here. The salivary diverticula, it may be 

 Avell to note, are parts of the stomodseum, whilst the glandular 

 caeca, ducts, and tooth-like hardenings, which belong to the 

 sexual organs very generally, are developed from the proc- 

 todaeum just as similar parts belonging to the mouth develop 

 from stomodseum. 



