CHAPTER XV. 



THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 



No other system in the body presents such peculiarities of development as 

 the urogenital system. In the first place, it is exceedingly complicated on ac- 

 count of its many parts. It is derived from both mesoderm (mesothelium and 

 mesenchyme) and entoderm. The urinary portion develops into a great com- 

 plex of ducts for the carrying off of waste products. The genital portion in 

 both sexes becomes highly specialized for the production and carrying off 

 of the sexual elements. In the second place, instead of one set of urinary organs 

 developing and persisting, three sets develop at different stages. The first 

 set (the pronephroi) disappears in part, but leaves certain structures which are 

 used, so to speak, in the development of the second. The second set (themeso- 

 nephroi) disappears for the most part, leaving, however, some portions which 

 are taken up in the development of the genital organs and other portions which 

 persist as rudimentary structures in the adult. The third set (the metanephroi 

 or kidneys) develops in part from the second and in part is of independent 

 origin. These conditions afford one of the most striking examples of the repe- 

 tition of the phylogenetic history by the ontogenetic, or, in other words, of von 

 Baer's law that an individual, in its development, has a, tendency to repeat its 

 ancestral history; for the first and second sets of urinary organs in the human 

 embryo represent systems that are permanent in the lower Vertebrates. In the 

 third place, the ducts of the genital organs are not homologous in the two sexes. 

 In the male the ducts (deferent duct, duct of the epididymis, efferent ductules) 

 are derived from the second set of urinary organs; in the female they (the 

 oviducts) are derived from other ducts which develop in the second set of 

 urinary organs, but which are not functionally a part of the latter. 



THE PRONEPHROS. 



The pronephros, with the pronephric duct, is the first of the urinary organs 

 to appear. In embryos of 2-3 mm. there are two pronephric tubules on each 

 side, situated at the level of the heart. Although their mode of origin has not 

 been observed in the human embryo, it is probable, judging from observations 

 on lower Vertebrates, that they arise as evaginations of the mesothelium. The 

 part of the mesothelium involved is that adjacent to the intermediate cell mass 

 (Fig. 343) . (The intermediate cell mass is the portion of the mesoderm interven- 

 ing between the primitive segments and the unsegmented parietal and visceral 



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