DEVELOPMENT. 



819 



From the lower end of the Wolffian duct a diverticulum grows back 

 along the body of the embryo toward its anterior extremity, and ulti- 

 mately forms the ureter. Secondary diverticula are given oft from it 

 and grow into the surrounding blastema of blood-vessels and cells. 



Malpighian bodies are formed just as in the Wolffian body, by the 

 invagination of the blind knobbed end of these diverticula by a tuft of 

 vessels. This process is precisely similar to the invagination of the pri- 

 mary optic vesicle by the rudimentary lens. Thus the kidney is devel- 

 oped, consisting at first of a number of separate lobules; this condition 

 remaining throughout life in many of the lower animals, e.g. , seals and 

 whales, and traces of this lobulation being visible in the human foetus at 

 birth. In the adult all the lobules are fused into a compact solid organ. 



Fig. 514. Section of intermediate cell-mass on the fourth day. m, mesentery; L, somato- 

 pleure ; a, germinal epithelium, from which z, the duct of Miiller, becomes involuted ; a, thick- 

 ened part of germinal epithelium in which the primitive ova O and o, are lying; E, modified 

 mesoblast, which will form the stroma of the ovary; WK, Wolffian body; y, Wolffian duct; X 

 160. (Waldeyer.) 



The supra-renal capsules originate in a mass of mesoblast just above 

 the kidneys; soon after their first appearance they are very much larger 

 than the kidneys (see fig. 516) , but by the more rapid growth of the 

 latter this relation is soon reversed. 



The first appearance of the generative gland has been already de- 

 scribed : for some time it is impossible to determine whether an ovary 

 or testis will be developed from it; gradually however the special char- 

 acters belonging to one of them appear, and in either case the organ 



