320 AN INTRODUCTION TO ZOOLOGY 



The . kidneys in the male lie in the abdominal cavity 

 attached to the latero-dorsal wall of the coelom, and as has been 

 noticed in dealing with their vascular supply, the right kidney lies 

 somewhat in front of the left. Each kidney is a flattened ovoidal 

 body of dark red colour with a distinct notch, the hilus, on its inner 

 surface marking the point at which the blood-vessels join it and the 

 ureter leaves it. On cutting off the ventral half of the kidney it 

 will be seen that the substance of which it is composed is distinguish- 

 able into two portions. The outer part, or cortex, is somewhat 

 granular in appearance and marked by a series of small spots ; it is 

 composed mainly of the much looped and twisted secretory portions 

 of the uriniferous tubules, and the spots indicate the positions of the 

 Malpighian bodies. The inner part, or medulla, is seen to be striated 

 and it consists almost entirely of the collecting parts of the tubules 

 which are nearly straight and cut lengthwise. It is drawn up into 

 a pointed pyramidal projection, termed the pyramid, upon the blunt 

 apex of which the tubules open. Inside the hilus is a crescentic 

 cavity, the pelvis of the kidney, which represents the dilated ex- 

 tremity of the ureter, and lies within the body of the gland. The 

 ureters are long, fairly narrow tubes passing along the dorsal body 

 wall to enter directly into the bladder at the level of the front end 

 of the pelvic symphysis. The urinary bladder itself is a large 

 dilatable, muscular, thin-walled sac in which the urine is stored as it 

 is secreted, and can be emptied at the will of the animal. The path 

 from the bladder will be considered when treating of the repro- 

 ductive organs, but it should be noted in passing that the ureters 

 in Lepus open directly into the bladder, and not into the cloaca as 

 in Rana. We find in the embryo, however, that they originally 

 open into the dorsal wall of the rectum, from the opposite side of 

 which is given off the ventral outgrowth destined to become the 

 bladder. In the course of development, however, as the urogenital 

 opening becomes separated from the end of the alimentary canal, 

 the openings gradually migrate round the rectum until they pass 

 on to the dorsal wall of the bladder itself. 



During the embryonic history of the excretory organs two other 

 kidneys make their appearance ; the first is the pronephros, and 

 the second is the mesonephros or Wolffian body, but both of these 

 disappear save for certain remnants which, however, lose their 

 excreting function and become related to the reproductive organs. 

 The functional kidney in the adult is the third to appear, and it is 

 termed the metanephros. It differs from those preceding it in that 

 its tubules are never segmentally arranged, but all open together 

 at one point ; consequent upon this we find that from the very 

 beginning its tubules are not related to the posterior segments of the 



