642 



Comparative Morphology of Chordates 



kidney is externally more or less deeply subdivided and therefore is 

 described as "tabulated." The kidneys are more or less conspicuously 

 lobulated in some ungulates — e.g., ox, rhinoceros, elephant — and in 

 some carnivores — e.g., bear, otter (Fig. 488), seals, walrus. The number 

 of lobules in the kidney of a walrus is estimated at between 300 and 

 400. Lobulation reaches its extreme in cetaceans, "each kidney appear- 

 ing to be an aggregation of a multitude of small kidneys closely packed 

 and contained within a single envelope" (A. B. Howell: "Aquatic 



Fig. 187. A diagram illustrating the repeated 

 branching of the collecting tubules in a nine-week 

 (30 mm.) human embryo. Roman numerals indicate 

 successive branchings. The diagram shows also the 

 origin of excretory tubules from the nephrogenic 

 tissue. (Redrawn from Rraus, after Kampmeier. 

 Courtesy, Neal and Rand: "Chordate Anatomy," 

 Philadelphia, The Blakiston Company.) 



Mammals"). The number of lobules in a kidney of the porpoise is 

 estimated at above 400 (R. Owen). 



In some cases, and especially in the^maller kidneys, the embryonic- 

 lobules may become closely compacted so that externally the adult 

 kidney shows little or no sign of lobules. Internally, however, they may 

 be recognizable. The human kidney is an example of this concealed 

 lobulation, the number of internal lobules, corresponding to the 

 "pyramids" (Fig. 85), ranging from 3 to 20. In many mammals, 

 especially the smaller (e.g., the cat), the kidney is simple, consisting 

 of a single lobe — therefore not "lobulated." 



The smaller kidneys, not lobulated or with concealed lobulation, 

 usually have the form of an indented ellipsoid — i.e., bean-shaped (and 

 a certain variety of bean is called "kidney-bean"). The common term 

 "kidney-shaped" refers to this characteristic form, but some mam- 



