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 durir.g 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- 



O. t. a. 



Fig. 348. — Diagram representing certain persistent portions of the mesonephros 

 in the female (see table). 

 Epo. I., Longitudinal duct of the epoophoron; Epo. t., transverse ductules of the epoophoron; O. t. a., 

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

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



live processes consist of (1) 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 jsince 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 acconT- 

 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 



Cephalic part 

 Caudal part 



Duct of mesonephros 



Efferent ductules 

 (vasa efferentia) 



Paradidymis 

 Vasa aberrantia 

 ' Deferent duct 

 Ejaculatory duct ' 

 Seminal vesicles 



Epoophoron 

 Paroophoron 

 Gartner's canals .- 



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

 embryos of ail higher Vertebrates, can be understood only by referring to the conditions in the 

 lower Vertebrates. In the majority of the Fishes and in the Amphibia the mesonephroi con- 

 stitute the functional urinary organs of the adult and possess essentially the same structure as 



