RESUME OF URINOGENITAL SYSTEM. 513 



tubules of the kidney, and ventrally (2) the oviduct or Miillerian 

 duct in the female, and the rudiments of this duct in the male. 

 In the female the formation of these ducts takes place by a nearly 

 solid rod of cells, being gradually split off from the ventral side 

 of all but the foremost part of the original segmental duct, with 

 the short undivided anterior part of which duct it is continuous 

 in front. Into it a very small portion of the lumen of the original 

 segmental duct is perhaps continued (PL 21, fig. i A, etc.). The 

 remainder of the segmental duct (after the loss of its anterior 

 section and the part split off from its ventral side) forms the 

 Wolffian duct. The process of formation of the ducts in the 

 male chiefly differs from that in the female in the fact of the 

 anterior undivided part of the segmental duct, which forms 

 the front end of the Miillerian duct, being shorter, and in the 

 column of cells with which it is continuous being from the first 

 incomplete. 



The tubuli of the primitive excretory organ undergo further 

 important changes. The vesicle at the termination of each 

 segmental tube grows forwards towards the preceding tubulus, 

 and joins the fourth section of it close to the opening into the 

 Wolffian duct (PI. 21, fig. 10). The remainder of the vesicle 

 becomes converted into a Malpighian body. By the first of 

 these changes a connection is established between the successive 

 segments of the kidney, and though this connection is certainly 

 lost (or only represented by fibrous bands) in the anterior 

 part of the excretory organs in the adult, and very probably 

 in the hinder part, yet it seems most probable that traces of 

 it are to be found in the presence of the secondary Malpighian 

 bodies of the majority of segments, which are most likely 

 developed from it. 



Up to this time there has been no distinction between the 

 anterior and posterior tubuli of the primitive excretory organ 

 which alike open into the Wolffian duct. The terminal division 

 of the tubuli of a considerable number of the hindermost of these 

 (ten or eleven in Scyllium canicula), either in some species 

 elongate, overlap, and eventually open by apertures (not usually 

 so numerous as the separate tubes), on nearly the same level, 

 into the hindermost section of the Wolffian duct in the female, 

 or into the urinogenital cloaca, formed by the coalesced terminal 



