THE INTERRELATIONS OF THE MESONEPHROS, 

 KIDNEY AND PLACENTA IN DIFFERENT 



CLASSES OF ANIMALS 



JOHN LEWIS BREMER 



FroiN iJic Drpdrhnciit of Anato))!!/, Harvard Medical School 



TWf:LVE FIGURES 



111 most aiiuiiiiiia the luesoiicphros begins its activity while the pro- 

 nephros is still at its functional height. For a time both organs func- 

 tion together; then the pronephros undergoes degeneration and the 

 mesonephros becomes the only functional excretory organ. This en- 

 tire complex of jirocesses has without further consicloration l)eeii trans- 

 ferred to the relation between the mesonephros and the metanephros, 

 and the question whether the mesonephros is actually functional in 

 the amniota has never been seriously considered. Weber ('97) was the 

 first to take it up and he endeavored to answer it in the following man- 

 ner. He compared the time of degeneration of the mesonephros with 

 that of the development of the metanephros; when he found that the 

 mesonephros degenerated before the metanephros could exercise an 

 (^xcretory function, he has assumed that the mesonephros did not 

 function, for if it had Ijeen active and had then degenerated before the 

 metanephros had begun its activity, there must have been a certain 

 period of development during which there was no excretion. Let us 

 adopt this same method in considering the special case of the func- 

 tion of the human mesonephros. Already in an embryo of 19.4 mm. 

 greatest length the majority of the mesonephric tulniles are so far in 

 process of degeneration that they cannot be regarded as having an 

 excretory function. Of the 35 tubules of this embryo only four are 

 actually still intact. In an embryo of 22 mm. greatest length none of 

 the mesonephric tul)ules were capable of functioning; in all the tubulus 

 secretorius had separated from the tubulus collectivus. If one inquires 

 how far the develo]oment of the metanephros has progressed at this 

 time, one finds that embryos of 22 mm. have just reached the anlage 

 of the second generation of uriniferous tubules. The first generation, 

 however, has as yet no fully formed Malpighian corpuscles. If, then, 

 the mesonephros had functioned as an excretory organ, there must 

 necessarily have been an interruption of this function on its degenera- 

 tion. Consequently, I regard the question as to the functioning of 

 the mesonephros as settled; it does not function as an excretory organ. 

 This does not. of couivse, imply that it may not have been active in 

 another manner unknown to us. 



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