188 THE STRUCTURE OF MAN 
sequently in the ancestors of Man, each of these organs once 
constituted in turn a permanent urinary system.! 
[The definitive kidney and ureter of Mammals arises at a com- 
paratively later period (eleventh to twelfth day of intra-uterine 
life) in relation to an outgrowth of the base of the mesonephric 
duct.2. This kidney, by extension, reaches to the mesonephridial 
region. On account of its distinct origin from the rest of the 
excretory system it 1s generally termed the metanephros, and its 
duct the metanephric duct. ] 
The definitive adult kidney of Man is, as a rule, a compact 
organ, with smooth walls; but its surface is not infrequently 
more or less distinctly furrowed, and thus apparently lobed. 
Lobation of the kidney is characteristic of certain lower Mammals 
[e.g. Cetacea and Ungulata]. The regular appearance of furrows 
in the kidney of the human embryo, giving rise to the so-called 
“renculi,” and the not infrequent occurrence of an increased 
number of renal arteries, justify the conclusion that the lobate 
structure may have been typical of the ancestors of Man. 
It is not yet evident what first led to the degeneration of 
the pronephros and to the loss of a renal function by the 
mesonephros in the amniota. So far as the mesonephros was 
concerned, the degeneration did not originally affect the whole 
organ, but only a part of it. The remainder, undergoing a change 
of function, became secondarily related to the male reproductive* 
apparatus. It gave rise with its duct to the epididymis and 
vas deferens, and became otherwise transformed into a series of 
vestigial appendages to the urinogenital organs of both sexes. 
1 This view, so far as it involves the conclusion that the mesonephros of the 
Amniota is the representative of the excretory organ of their ancestors, receives 
its chief support from the condition of the excretory apparatus in Reptiles. These 
animals pass through a period in which the greater part of the mesonephros con- 
tinues functional, side by side with the later definitive kidney. In the Lizards, for 
example, it shrivels up after the first hibernation, 7.e. in the second year. 
* [The metanephric tubules of Mammals are stated to arise as outgrowths of this 
diverticulum itself, but in other animals there is good reason for regarding them as, 
at any rate in part, distinct in origin—7.e. as arising independently of the duct 
with which they subsequently become connected, in the manner typical of the meso- 
nephric series. The recent researches of Semon (Jenaische Zeitschrift, Bd. xxvi. 
p. 89) and Field (Bullet. Mus. Comp. Zool. Harvard, vol. xxi. p. 201) have revealed 
striking details of similarity in development between the pro- and meso-nephridia, 
rendering it more difficult than hitherto sharply to discriminate between them. 
Indeed, recent discovery tends to suggest that the pro-, meso-, and meta-nephridia 
are portions of one continuous system, and that their apparent independence is due to 
the assumption of secondary relationships with independently formed ducts. ] 
3 The initial stages in this process have been permanently retained as the adult 
condition by the Elasmobranchs and Amphibia. ; 
