VERTEBRA TA. 355 



into the Wolffian or mesonephric duct in other types (Teleostei and Ga- 

 noidei ?). These tubules are arranged segmentally, one to each segment 

 in the embryo Elasmobranch, and the adult Myxine, and Gymnophiona, but 

 in other types they are, especially in the posterior segments of the body, 

 more numerous than the segments. Secondary and tertiary tubules are 

 added to those first formed. Each tubule is typically composed of (i) a 

 ciliated funnel or nephrostome opening into the coelome ; (2) a Malpighian 

 body, i. e. a dilatation with a tuft of blood-vessels projecting into it : (3) a 

 coiled glandular tube or tubulus uriniferus ; and (4) a collecting tube. The 

 nephrostomata persist in Elasmobranchii and Amphibia in connection with 

 more or fewer of the primary tubules. Present in the embryo in many 

 other instances, they are always aborted at a certain stage of growth. The 

 anterior part of the mesonephros becomes connected with the testis in the 

 male of Elasmobranchii, certain Ganoidei (? all), all Amphibia and Amniota. 

 The corresponding portion in the female is modified or nearly aborted in 

 the Ichthyopsida, and persists in Amniota as a variable rudiment (ep- 

 oophoron of Mammalia). The posterior non-sexual part of the mesone- 

 phros becomes partially or wholly independent of the fore part in Elasmo- 

 branchii and communicates with the Wolffian duct by one or more 

 independent ureters, specialised collecting tubes. In Amphibia it remains 

 continuous with the sexual region, but is strongly marked off from it in 

 Urodela ; and its ducts sometimes tend to unite inter se before they fall 

 into the Wolffian duct. The permanent kidney of the Amniota is a meta- 

 nephros. It appears to be ' a specially differentiated posterior section of the 

 mesonephros ' (Balfour) developing at a later period, comparable to the pos- 

 terior region of the mesonephros of Elasmobranchii. The ureter in this case 

 is an outgrowth of the Wolffian duct, the collecting tubes of the ureter ; the 

 gland tubes (tubuli uriniferi) and Malpighian bodies are formed indepen- 

 dently (?) out of the mesoblast. There are no nephrostomata. Remnants 

 of the non-sexual part of the mesonephros may persist (par-epididymis, 

 par-oophoron, of Mammalia). 



The primitive connection of the segmental duct or of its derivatives, 

 the Miillerian and Wolffian ducts, as well as of the ureter in Amniota with 

 a cloaca common to both anus, urinary and genital ducts, is retained in 

 Cyclostomi, Elasmobranchii, and Dipnoi among Pisces, in all Amphibia and 

 Sauropsida and in the Prototheria among Mammalia. The cloaca is divided 

 into an anal, section and a urogenital section in all Ganoidei, many Teleostei 

 and nearly all Mammalia. In Holocephali and a few Teleostei among 

 Pisces, and a few female Mammalia the anal, genital and urinary apertures 

 are independent. The anal aperture of Mammalia is always placed behind 

 the genito-urinary apertures, in front in all the other classes. But in all 

 Vertebrata the urinary aperture is anterior to the genital. The ureters in 

 Mammalia, except Prototheria, open into a urinary bladder, a persistent 



A a a 



