MORPHOLOGY OF CYCLOSTOMATA. 393 



ticula. The vessels in the cranial as well as in the caudal part 

 of the pronephros are weaker than those in the middle (the third 

 and fourth) ; only the latter vessels develop further and become the 

 vitelline artery. Van AYyhe confirms Rückert's account and 

 has described three vessels in Friüluriis. In addition to these, 

 VAN Wyhe has pointed out the very small segmental vessels on 

 the left side, which go not to the intestine, but to the body- 

 wall. They are not equivalent to Ihe intestinal vessels on the 

 opposite side. One of them gives a branchlet to the glomerulus 

 which sends out, in its turn, a branchlet to the cardinal vein. 

 The homologous vessels on the right side are to be seen coming out 

 of the root of the vitelline artery. Boveri remarks that the 

 vessels of Paul Mayer present many points of harmony with 

 the branchial vessels in Amphioxus. Rabl agrees essentially wnth 

 the account given by Rückert and van Wyhe, but denies the 

 existence of a glomerulus. According to Rabl, the structure 

 called the glomerulus by Rückert does not fulfil the condi- 

 tions of being a glomerulus ; he says : " Eine einfache Ausbucht- 

 ung einer Arterie ist noch keine Gefässschlinge, geschweige 

 denn ein Glomerulus " {loco cit., p. 668). 



Most of the early investigators, who observed the develop- 

 ment of the Teleostian pronephros, believe it to be mesoblastic 

 in origin. There are very few writers as Ryder ('87), and Brook 

 ('88), who derive the segmental duct from the epiblast. According 

 to Oellacher ('73), Goette ('75 and '88), Fürbringer ('78), 

 and Hoffmann ('86), the first Anlage of the pronephros is 

 brought about by the evagiuation of the parietal layer of the 

 mesoblast at the level of the junction of the somite with the 

 lateral plate, formiug thus a longitudinal groove on each side. 



