440 The American Naturalist. [May, 



Finally, in the stage to which all Craniata attain at the present day, a 

 second generation of tubules, the mesonephric tubules, is cut off from 

 the primary pronephric tubules, while the Malpighian capsules divide 

 into dorsal and ventral parts. Into the first open the new mesone- 

 phric, into the ventral the primary pron p] ric tubules, each by its 

 internal funnel, while its external funnel opens into the general coe- 

 lom. The appearance of this dorsal mesonephros is accompanied by a 

 reduction and transformation of the pronephros, combined with sym- 

 pathetic ganglia (suprarenal bodies) to form the adrenal body. The 

 reproductive cells must now pass out by the new mesonephric tubules 

 to the pronephric duct. 



In male vertebrates there is in addition a reduction of the germinal 

 epithelium to a short portion of its primitive length and a correspond- 

 in- restriction of the connection of testis and nephros. This leads to 

 the distinction between "sexual" nephros and the more posterior 

 " pelvic " nephros, which is finally quite separated in the higher forms 

 as the metanephros or true kidney. 



The complete separation of testis and excretory organs in the tele- 

 osts and cyclostomes is to be regarded as secondary. 



In the female, secondary changes, perhaps connected with increase 

 in size of the ova, have led to the restriction of the egg-conducting 

 function of the pronephros to a single tubule, which then opened into 

 a secondary duct, the Miillerian duct, divided off from the pro- 

 nephric duct. For the understanding of the true value of the perito- 

 neal opening of the Miillerian duct and for any satisfactory homolo- 

 gizing of the oviducts of teleosts we must await further researches upon 

 these organs in the ganoids. 



True abdominal pores, serving for exit of the reproductive [.redacts 

 in Cyclostomes but not in any Gnathostomes, arc to be regarded as 

 having probably taken that function secondarily. Their morphologl 

 cal value remains unknown: functionally they may perhaps be 

 brought into relation with the need of communication of the coelom 

 and external water to equalize pressure in deep sea forms. 



In many vertebrates the original function of the external nejdinc 

 funnels, those opening into the large coelom as contrasted with the 

 internal funnels opening into the Malpighian capsule.*, that is the 

 passing out of water from the coelom, has been assumed more and 

 more by the glomeruli and thus the external funnels have been 

 abandoned as useless. Along with this loss of the coelom funnels 

 there has been a loss of cilia in the tubules, in many forms, since 



