DEVELOPMENT OF THE UROGENITAL ORGANS. 1143 
even in the adult, traces of its original subdivision into lobules. Each lobe corr esponds 
to a medullary pyramid of the adult and its surrounding cortex. As the ureter in- 
creases in length, it becomes separated from 
the Wolfhan duct, and acquires a distinct 
opening into the anterior part of the cloacz 
nearer the head of the embryo than that of 
| the Wolffian duct. This part of the cloaca 
| receiving the ureters becomes the bladder. 
Bladder.—The bladder is formed from 
the upper part of the anterior subdivision 
of the cloaca (Figs. 780 and 781), which 
increases in width as the openings of the 
ureters become shifted further from one 
another. Inferiorly the anterior part of the 
cloaca remains narrow, and is continuous 
with the upper part of the urogenital canal. 
At a very early stage the triangular sur- 
face, known in the adult as the trigonum 
| vesice, is to be recognised. From _ the 
cephalic end of the developing bladder the 
7 allantoic canal extends to the umbilicus ; but 
| the allantois loses its lumen in the fifth week, 
and the portion enclosed within the body of 
~the embryo, and known as the urachus, be- 
comes converted into the fibrous cord found 
in the adult connected with the bladder apex. 
The cavity of the allantois is sometimes 
not lost so early, and in rare cases it has been 
found persisting in the child or adult as a 
pervious channel extending from the apex of 
the bladder to the umbilicus. Here it may 
open on the surface of the body. 
By some authorities the bladder is stated 
to arise as a dilatation of the proximal part 
of the allantois rather than from the cloaca 
itself. The distinction, however, does not 
seem to, be important, and probably both 
the cloaca and the allantois share in its 
formation. 
| Male Urethra.—The urethra in the 
male is composed of two parts which are 
developmentally distinct. One of these corre- 
sponds to the prostatic and membranous 
portions of the adult, and owes its origin to 
| the endodermal cloaca, while the other part 
| corresponds to the spongy portion of the adult, Fic. 782.— DiacramMatic REPRESENTATION AND 
. and arises in connexion with the ectodermal COMPARISON OF THE MANNER IN WHICH THE URO- 
cloacal fossa and sexual eminence (Fig. 7 82). GENITAL PASSAGES ARISE IN THE TWO SEXES. 
The upper part of the prostatic urethra es an In each case the hypoblast is indicated by a thick 
| origin similar to that of the bladder, and is black line, while the epiblast is shown by a double 
5 5 : line with cross shading. 
. derived from the upper portion of the uro- | nit 
genital subdivision of the cloaca ; the lower 4, Represents a stage passed through by both sexes, 
5 : ’ and illustrates the connexions of the urogenital 
prostatic and the membranous parts are ducts and of the rectum with the ectodermal cloacal 
formed from the urogenital canal of the embryo. __ fossa ; gd, indicates the position where the Wolffian 
The spongy portion of the adult urethra and Mullerian ducts open into the urogenital canal ; 
ge, genital eminence ; ugs, urogenital canal. 
B. Male condition. The greater portion of the urethral canal is ectodermal in origin, and is derived 
| from the ectodermal cloacal fossa and the groove on the genital eminence by the meeting of their 
lateral boundaries. (The portion of the canal which is usually the last to be closed in is 
; indicated at w.) The upper part of the urethra is hypoblastic in origin. The position where the 
: genital ducts (in the male represented by the vasa deferentia and uterus masculinus) join the urogenital 
. canal is indicated at vd ; s, symphysis pubis. 
C. Female condition. Owing to the shortening of the urogenital canal (wgs, in A) the urethral and 
genital openings come to lie in the bottom of the urogenital fissure. The deepest part of this fissure, 
immediately surrounding the openings, is formed by hypoblast derived from the urogenital canal. 
