MUSEUM OF COMPARATIVE ZOOLOGY. 303 



of her offspring without calling into play their individual activities. In 

 the former class may be reckoned Cyclostomes, Teleosts, Ganoids, Dip- 

 noi, and Amphibia. 1 Omitting from consideration the little known 

 Dipnoi, a functional pronephros appears in all the members of this group 

 without exception, and is most highly developed in those forms (Petro- 

 myzon, Amphibia) which pass through a protracted larval stage. The 

 other class includes Selachians, Reptiles, Birds, and Mammals. In every 

 member of this group the pronephros is rudimentary. 



/ conclude, therefore, that pronephros and mesonephros are parts of one 

 ancestral organ; that the glomeruli are strictly homodynamous ivith the 

 glomus ; that the entire tubular portion of the pronephros is represented in 

 the mesonephros ; that the cavity of a Malpighian capsule and the nephro- 

 stomal canal connecting it ivith t/ie body cavity are detached portions of the 

 coelom, the equivalents of which are not thus differentiated in the pronephros ; 

 that the pronephros is developed as a larval excretory organ ; and that the 

 period at which it appears largely accounts for its peculiarities of structure. 

 This general conclusion, which is mainly based upon a study of the con- 

 ditions in Amphibia, is, in my opinion, in perfect harmony with the 

 recorded observations on other groups. 



It must be remembered in this connection, however, that the pro- 

 nephros may possibly have been developed from the primitive excretory 

 organ independently in two or more groups, in response to similar physio- 

 logical necessities. While I have not been able to preclude this possibility, 

 I am nevertheless inclined to the opinion that in general a closer relation 

 exists, and that consequently the pronephros is homologous throughout 

 all Vertebrates. An interesting condition manifests itself in those forms 

 (Teleosts and Ganoids) in which the pronephros remains functional until 

 the individual is nearly adult. In these the pronephric chamber becomes 

 partially (Lepidosteus) or entirely (Teleosts) cut off from the body cavity 

 and comes to resemble an enormous Malpighian capsule. The region in 

 Crocodilus intermediate between pronephros and mesonephros shows a 



1 In the one group, the eggs are holoblastic, or if meroblastic contain little yolk 

 (Teleosts); in the other, they contain much yolk, or the young are nourished by 

 means of a placenta (.Mammals). Mr. Samuel Garman has kindly called my atten- 

 tion to a number of cases in Amphibia where the period of larval life is greatly 

 reduced. The occurrence of holoblastic segmentation in this group appears to 

 me to afford adequate evidence that such conditions are secondary. Moreover, 

 there actually appears to be a reduction of the pronephros in such species as 

 abandon in part their larval life. In the case of Hylodes martinicensis, mentioned 

 by Mr. Garman in this connection, Selenka ('82) has shown the pronephros to 

 be very degenerate. 



