CHAPTER XIII 
THE URINOGENITAL SYSTEM 
Tue history of the pronephros and the origin of the meso- 
nephros have been already described (Chap. V). We have now 
to consider (1) the later history of the mesonephros, (2) the 
development of the metanephros or permanent kidney, (3) the 
development of the reproductive organs and their ducts, and 
(4) the development of the suprarenals. All these organs form 
an embryological unit, by virtue of their mode of origin and their 
interrelations. Thus we find that the intermediate cell-mass is 
significant for the development of all: its growth causes the forma- 
tion of the Wolffian body, on the median face of which the gonads 
arise. The secreting tubules and renal corpuscles of the perma- 
nent kidney are also derivatives of the intermediate cell-mass. 
The Wolffian duct is derived from the same source, and by change 
of function becomes the vas deferens, after functioning for a while 
as the excretory duct of the mesonephros. Certain parts of the 
mesonephros also enter into the construction of the testis. And 
the Millerian duct, which forms the oviduct of the female, is 
derived from the epithelium covering the Wolffian body. 
I. Tuer Larter History oF THE MESONEPHROS 
In Chapter VI we traced the origin of the nephrogenous 
tissue, and the differentiation of the first mesonephric tubules 
within it. We saw that in each of the segments concerned a 
number of balls of cells arises by condensation within the neph- 
rogenous tissue, and that these become converted into vesicles. 
We saw also that each vesicle sends out a tubular sprout from 
its lateral side to the Wolffian duct, with which it unites; and 
that its median face becomes converted into a renal corpuscle. 
These processes take place sucessively in antero-posterior order 
within the somites concerned, so that a series of stages in the 
development of the tubules may be studied in the same embryo. 
Moreover, all the tubules of a given somite do not develop simul- 
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