THE FROG 75 



of the renal artery, which enters the capsule, breaks up into a mass 

 of capillaries, and unties again to form a single vessel still to be 

 regarded as an artery. From the Malpighian body the tubule runs 

 fairly straight dorsally, and becomes much coiled near the dorsal 

 surface. It then runs ventrally, again becomes coiled, and finally 

 passes dorsally once more to open into a collecting tubule, extending 

 nearly transversely beneath the dorsal surface of the kidney. Thus 

 in a transverse section the middle portion consists of tubules cut 

 more or less longitudinally, and on the dorsal and ventral sides of 

 this is a strip where the coiled parts of the tubes are cut in all sorts of 

 directions. The ventral border is easily recognised by the presence 

 of the conspicuous Malpighian bodies, and also because the nephro- 

 stomes are situated on this side. The nephrostomes are ciliated 

 funnels, that originate in connection with the urinary tubules, and 

 are to be found as such in the young tadpole. During the course 

 of development they lose their connections and acquire new ones 

 with branches of the renal veins, hence serving as a means of com- 

 munication between the lymph in the sub-vertebral sinuses and the 

 circulatory system. The urinary tubules are lined by a ciliated 

 epithelium, whose character varies slightly in different parts. The 

 collecting tubule opens into a longitudinal canal on each side ; that 

 on the inner edge of the kidney is called Bidder's canal, and that on 

 the outer side is of course the ureter. The vasa efferentia open into 

 Bidder's canal, and the sperms are conveyed thence by the collecting 

 tubules to the ureter, so that they do not enter the uriniferous tubules. 

 As already noticed, blood is brought to the kidneys by the renal 

 portal vein, and this vessel breaks up into dilated capillaries, sinusoids, 

 whose walls are closely apposed to the uriniferous tubules. The 

 artery coming from Bowman's capsule, also bringing blood to the 

 kidney, breaks up into capillaries that open into the sinusoids. 

 Blood is carried away from these vessels by factors of the renal 

 veins. It appears probable that the function of the capsule is to 

 remove excess of water from the blood, while the nitrogenous waste, 

 in the form of urea, is taken from it by the tubules. 



The urinary bladder is lined by a peculiar type of epithelium, 

 known as transitional epithelium. The cells are only two or three 

 deep, fit together irregularly, and are not arranged in definite layers 

 one above the other, as in stratified epithelium. Outside the epithe- 

 lium is a muscular layer, composed of numerous strands of non- 

 striate muscles distributed in an irregular manner, so as to form a 

 network in a thin layer of connective tissue. The whole is enclosed 

 in a close fitting extension of the peritoneum. 



