xv KIDNEYS 399 



come to open into the archinephric duct, although they do not in 

 any way contribute to its formation (Fig. 229, B). Segmentally- 

 arranged twigs from the dorsal aorta end in tufts of capillaries 

 or glomeruli, each of which projects into a small sac-like enlarge- 

 ment of a mesonephric tubule, pushing, before it the wall of the 

 sac. In this way a double- walled " Malpighian body," contain- 

 ing a " glomerulus," is formed in connexion with each tubule. 

 Subsequently, the mesonephric tubules increase in number by 

 budding. Xew nephrostomes and Malpighian bodies are developed 

 on the secondary branches, and the original segmental arrange- 

 ment of the tubules becomes obscured. With the growth of 

 new tubules, and the formation of blood-vessels and of connective 

 and lymphoid tissues between them, each mesonephros finally 

 assumes the condition of a compact gland imbedded in the dorsal 

 wall of the coelom, with its ventral surface invested by the 

 peritoneum. A " metanephros," which in the higher Vertebrates 

 replaces the mesonephros as the functional kidney, is perhaps 

 not represented in Fishes. 



A more or less well-developed pronephros is present in the 

 embryos or larvae of the Cyclostomes and of all Fishes, but as a 

 rule it completely disappears at an early period and is replaced 

 by the mesonephros. It is retained throughout life, however, 

 in the Myxinoid Cyclostomes (Fig. 230, B), and has its persistent 

 nephrostomes opening into the pericardial cavity. 1 In a few 

 Teleosts the pronephros is also persistent, as in Fierasfer and 

 Dactylopterus, and in others the organ may not completely dis- 

 appear until the approach of sexual maturity. But with these 

 exceptions the mesonephros is the sole functional kidney in the 

 adults of the Cyclostomes and of all Fishes. As regards the 

 nature of the duct by which the excretion of the mesonephros is 

 conveyed outwards, there are notable differences in different 

 Craniates. The Cyclostomes and the Teleostoini retain that part 

 of the archinephric duct into which the mesonephric tubules open, 

 and which remains after the atrophy of the pronephros (Fig. 230, 

 B, E, F). In Elasmobranchs, and probably also in the Dipnoi, 

 a special mesonephric duct is developed in a way which will 

 be described later (Fig. 230, C, D). In the males of Elasmo- 

 branchs some of the hinder mesonephric tubules unite to form 



1 W. Miiller, Jen. Zeitsch. ix. 1875, p. 107 ; Semon, Carl Gegenlaur s Festschrift ,. 

 Leipzig, 1896, hi. p. 169. 



