THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 



393 



purifying processes in its close contact with the tubules and is returned to the 

 heart by the posterior cardinals, or, after the cephalic ends of the latter atrophy, 

 by the subcardinals and the inferior vena cava (see p. 258; also Fig. 232, B). 

 There is thus present a true renal portal system, similar to the hepatic portal 

 system. 



Although the mesonephroi become large functional organs during the earlier 

 stages of development, they atrophy and disappear for the most part, coinci- 

 dently with the appearance and development of the kidneys. The degeneration 

 begins during the sixth or seventh week and goes on rapidly until, by the end of 

 the fourth month, little remains but the ducts and a few tubules. The degenera- 



Ovd. 



Epo. 



Epo. t. 



FIG. 348. Diagram representing certain persistent portions of the mesonephros 



in the female (see table). 



Epo. /., Longitudinal duct of the epoophoron; Epo. /., transverse ductules of the epoophoron; 'O. t. a., 

 ostium abdominale tubae; Ovd., oviduct; X represents a small duct which, if present, leads 

 from the epoophoron to one of the fimbriae of the oviduct. 



tive processes consist of (i) an ingrowth of connective tissue among the tubules, 

 (2) atrophy of the epithelium of the tubules, and (3) atrophy of the glomeruli. 

 The portions which remain differ in the two sexes, and since the remnants 

 are taken up in the formation of the male and female genital organs it seems 

 best to discuss them more fully under those heads (pp. 417, 420). The accom- 

 panying table, however, will give a clue to their fate (see also Figs. 347 and 

 348). A more comprehensive table will be found on p. 427. 



Male Female 



Mesonephros 



Duct of mesonephros 



Cephalic part 

 Caudal part 



f Epoophoron 

 s Paroophoron 



Efferent ductules 

 (vasa efferentia) 



Paradidymis 



Vasa aberrantia 

 I" Deferent duct 

 <j Ejaculatory duct 

 [ Seminal vesicles j 



TIic significance of the mesonephroi, which, as well as the pronephroi, are present in the 



embryos of all InVher Vertebrates, can be understood only by referring to the conditions in the 



lower V ;s. In the majority of the Fishes and in the Amphibia the mesonephroi con- 



actional urinary organs of the adult and possess essentially the same structure as 



Gartner's canals 



