330 Mr Kerr, The Genito-urinary Organs 
the ventral wall of the pronephric chamber, the glomus becomes 
later on by differential growth carried inwards and upwards so 
as to be attached to the median wall of the chamber close to the 
rudiment of the dorsal aorta. 
The myocoele never becomes more than a narrow split, but the 
perivisceral cavity extends with further development ventralwards 
of the pronephros. About stage 31 the perivisceral cavity forms 
a considerable chamber on each side, the glomus hanging freely 
into it from its medio-dorsal angle, and the pronephros forming 
a bulging inwards of its dorsal wall. In stage 32 the pronephric 
swelling comes into contact with the oesophageal rudiment and 
fuses with it so as to separate off a chamber lying above it, into 
which the nephrostomes open and which contains the glomus, 
from the perivisceral cavity. This fusion never extends round 
the posterior end of the pronephros, so that here the pronephric 
chamber remains freely open to the general coelom. 
During the period in which the cloacal aperture is temporarily 
closed the archinephric duct becomes greatly dilated, apparently 
distended by the continued forcing into it of coelomic fluid by 
the pronephrostomes. A similar dilatation of the cavities of the 
pronephros was observed in the case of the frog by Marshall and 
Bles, the obstruction to the outflow of fluid being in this case, 
however, caused by the occlusion of the archinephric duct itself. 
In Lepidosiren the prevention of the outflow of excretory products 
to the exterior is associated with the fact that during this period 
the larvae are lying crowded together practically motionless in the 
still water at the bottom of the nest. It is probably to be looked 
upon as an adaptation to diminish pollution of the surrounding 
water 1 . 
The substitution of the mesonephros for the pronephros as the 
functional renal organ is a gradual process which takes place in the 
stages before and during metamorphosis. 
Mesonephros. 
The mesonephros forms the functional kidney of the adult. 
The rudiments of its tubules are first noticeable about stage 30 as 
1 The enormous dilatation of the homologue of the Amphibian bladder seen in 
the allantois of the Amniota is probably an analogous phenomenon, the embryo 
being here for the first time completely shut up in a shell through which diffusion 
of fluid hardly takes place at all. The urinary secretion therefore, instead of being 
allowed to pass into the space round the embryo — thence to pass away by diffusion, 
is retained within the tissues of the embryo (both yolk sac and allantois being 
morphologically parts of the embryo, in spite of ordinary Text-book usage) in the 
homologue of the urinary reservoir of the anamniotic ancestral form. That the 
cloacal bladder of Amphibia — the homologue of the allantois of Amniota — is physio- 
logically a urinary bladder is indicated by an abnormal specimen of Rana temporaiia 
examined by me, in which the cloacal bladder was absent. In this specimen 
the whole of the rectum was greatly distended by accumulated urine. 
