DISCUSSION 439 



and a domed dorsal aspect. It lies between the lower poles of the kidney and is flanked 

 by the testes in the male and the ovary in the female. In ventral view its neck overlies 

 the vasa deferentia and the broad ligament which carries them in the male, and the 

 corpus uteri and vagina in the female. The two hypogastric arteries ascend on to the 

 latero-ventral face of the vesica and converge upon its ventral face (Figs, i, 12), 

 becoming surrounded by thick muscular sheaths completely apposed to one another 

 where the umbilicus meets the ventral body-wall. The ureters, after leaving the kidney, 

 pass backwards and inwards in an oblique direction towards the neck of the bladder. 

 During this part of their course they are completely hidden under the great fibrous pad 

 which occupies the dorsum of the inguinal region of the abdominal cavity. They emerge 

 from their peritoneal coverings anterior to the hypogastric arteries and turn ventrally 

 and medially on to the wall of the neck of the bladder. Upon the neck of the bladder 

 they proceed forwards for a very short distance, and then penetrate into the fibrous 

 investments to open into the bladder close to one another on the dorsal side of the cavity. 

 The openings of the ureter are oval and are separated by a longitudinal fold of mucous 

 membrane. During the straight part of the course of the ureters these ducts are lined 

 by a longitudinally folded cubical epithelium, continuous with that which lines the 

 main urinary duct of the kidney, and surrounded by a similar tough fibrous investment. 

 Where the ureters ascend, however, on to the neck of the bladder the longitudinal 

 folding of their lining epithelium is replaced by a folding of a more complex nature. 

 The folds become compound and each consists of a main longitudinal crest with regularly 

 spaced lateral subsidiary projections on each side of it, so that the canal becomes lined 

 by a number (about half a dozen) of pinnate ridges. 



The bladder itself has extremely thick muscular walls so that in the foetus the internal 

 cavity is comparatively very small. It is lined by a membrane, thrown into longitudinal 

 folds which disappear at about the level of the openings of the ureters, so as to leave the 

 lining of the neck smooth and unfolded. Near the verumontanum, as already described, 

 a median dorsal ridge rises up to form the crista urethrae. 



In the adult the bladder is very elastic and when empty or nearly so — as it always is 

 in whales drawn up on the flensing plan — it forms a comparatively very small sac, lined 

 by an abundantly folded membrane, at the posterior angle of the abdominal cavity. 



DISCUSSION 



It is not intended in this paper to deal at length with the much discussed question of 

 the relationships of the Cetacea to other groups of mammals. This question and that of 

 the origin of the groups has already been examined in detail by Weber (1886 and 1904), 

 Gregory (1910), Winge (1921), Kiikenthal (1922) and Anthony (1926). It is, however, 

 proposed to make comparisons between the urino-genital systems of both sexes of the 

 Cetacea and those of other mammals, in the hope that perhaps such comparisons may 

 throw light on the relationships of the order. 



It might be expected that the affinities of any group of animals would be shown forth 



