310 COMPARATIVE ANATOMY. 



the embrvo ; this condition may reimin throughout life (e.g., 

 Cetacea, Piimipedia, Ursus, Lutra, Bos), or the lobes may become 

 more or less completely united. In the latter case the original 

 division into lobes may still be recognised more or less plainly 

 internally. A section of the kidney shows an inner layer, the 



FIG. 243. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE KIDNEY OF A 



MAMMAL. 



R, 7?, cortical substance ; M, M, medullary substance arranged in pyramids (Pr) : 

 between the latter the cortical substance extends in the form of the columns of 

 Bertini (B, B} ; Ca, calyces ; PC, pelvis ; Ur, ureter. 



medullary substance, arranged in the form of wedges, 

 the urinary pyramids, and an outer layer, or cortical 

 substance, extending as the columns of Bertiui between the 

 pyramids (Fig. 243, R, B}. The pyramids correspond roughly to 

 the embryonic lobes of the kidney, though several lobes may fuse 

 together in one pyramid. 



The glomeruli as well as the coiled tubules, which are surrounded by a 

 network of blood-capillaries, lie in the cortical substance, while the so-called 

 straight tubules occur principally in the pyramids, where they gradually 

 anastomose to form larger collecting tubes. 



Concerning the morphological relations of the urinary bladder of 

 Mammals, comp. p. 274. 



GENERATIVE ORGANS. 



In Amphioxus the generative "glands" or gonads remain for 

 a long time in an undifferentiated condition as regards sex. They 

 have a marked segrnental arrangement, and each portion opens 

 separately into the peribranchial cavity, whence the generative 

 products pass to the exterior through the atrial pore. 



Fishes. Specially differentiated generative ducts arc wanting 

 in Cyclostomes, as well as in Eels, female Salmonidae, 

 and Laimargus borealis (an Elasmobranch). In these cases, 

 both sperm and ova are shed directly into the body-cavity, whence 

 they pass out through the abdominal pores (see p. 265.) 



