454 COMPARATIVE ANATOMY 



and Miillerian ducts, takes place, and a distinction between an 

 anterior, a middle, and a posterior section of the mesonephros may 

 be observed in Plagiostomes (Figs. 339, 349). In the male the 

 anterior portion (epididymis or parorchis) comes into connection 

 with the testis by means of small ducts, the vasa e/erentia, and 

 in the female undergoes reduction : the middle portion usually 

 becomes considerably reduced, especially in the female, but its 

 ducts still communicate with the Wolffian duct, which is more or 

 less dilated at its posterior end and in the male serves mainly as 

 a vas deferens : the large posterior portion forms the essential part 

 of the kidney, and empties its secretion by means of special urinary 

 ducts, some of which unite with one another, into the urinogenital 

 sinus. 



This differentiation of the hinder part of the mesonephros, 

 and the formation of special ducts in connection with it, seems, 

 in a sense, to foreshadow the condition which occurs in the 

 Amniota (p. 446). 



In the Holocephali no sexual portion of the kidney can be 

 distinguished, and the anterior part in the male Callorhynchus is 

 much larger and more massive than the posterior section, which, 

 as in Plagiostomes, is provided with special ducts. 



The kidney is very variable in form and size : its outer 

 border is usually notched, and this, together with the arrange- 

 ment of the nephrostomes in the embryo, points to the original 

 segmental arrangement of the organ. The segmental char- 

 acter, however, disappears later on ; in the adult the nephro- 

 stomes are much less numerous than the vertebrae of this region, 

 but their number and size vary much in different genera and even 

 in individuals, and they do not persist in all (e.g. Carcharias, 

 Mustelus, Echinorhynchus, Myliobates, Raia). 



Teleostomes. The larval pronephros of Ganoids consists of a 

 varied number of canals, the segmental arrangement of which has 

 so far not been accurately ascertained. 1 In the definitive kidney 

 (mesonephros) a segmental arrangement is recognisable. In 

 Sturgeons, its connection with the ccelome by means of nephro- 

 stomes takes place only after the individual canals have become 

 connected with the pronephric duct. 



The pronephros in the majority of Teleosts has only a 

 temporary significance, and extends over from one to five 

 segments. 2 The mesonephros constitutes the excretory organ of 

 the adult, and consists of a narrow band varying in size and 

 diameter in different regions, situated on the dorsal side of the 

 body-cavity, between the vertebral column and the swim-bladder. 

 Secondary fusions between the two kidneys often occur. 



1 The nephrostomes of the pronephros open either directly into the coelome 

 or into a space shut off from the latter which encloses the more or less folded 

 glomus (p. 443). 



2 It persists in Lepidogaster, Fierasfer, and Zoarces, 



