462 



COMPARATIVE ANATOMY 



kidney shows an inner layer, the medullary substance, arranged in 

 the form of wedges the urinary pyramids, and an outer layer, 

 or cortical substance, extending as the columns of Bertini between 

 the pyramids (Fig. 347). 



The Malpighian capsules as well as the coiled portions of the 

 tubules, surrounded by a network of blood-capillaries, lie in the 

 cortical substance, while the straight portions of the tubules extend 

 through the pyramids, where they gradually anastomose to form 

 larger collecting tubes. 



The urinary bladder in Monotremes and nearly all Marsupials 

 represents the whole allantois, and the ureters open at 

 its junction with the urinogenital canal (Fig. 359). In placental 



N.M 



FIG. 348. A, RIGHT KIDNEY OF A DEER ; B, KIDNEYS (N) AND ADRENAL 

 BODIES (JV.JV) OF THE HUMAN EMBRYO. Both from the ventral side. 



Ur, ureters. 



Mammals, on the other hand, the greater part of the bladder is a 

 new formation, which arises by a special differentiation of the 

 cloaca, the latter becoming divided into a dorsal and a ventral 

 portion by the formation of a septum. The former is continuous 

 with the rectum, and the latter gives rise to the urinogenital canal 

 and bladder, which is continuous distally with the stalk of the 

 allantois from which the urachus (p. 440) and the median ligament 

 of the bladder are formed. These two sections of the cloaca 

 gradually become further separated from one another by the 

 development of the perinceum, or space between the urinogenital 



