XV KIDNEYS 399 
come to open into the archinephric duct, although they do not in 
any way contribute to its formation (Fig. 229, B). Seomentally- 
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. New 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. 250, B), and has its persistent 
nephrostomes opening into the pericardial cavity." In a few 
Teleosts the pronephros is also persistent, as in Merasfer 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 Teleostomi 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 Gegenbaur’s Festschrift, 
Leipzig, 1896, iii. p. 169. 
