266 BULLETIN OF THE 



Turning now to Craniota, the pronephros in Amniota and Selachii 

 is a wholly degenerate structure ; in many Anamnia, however, it serves 

 for a longer or shorter time as a functional excretory organ. 



The" pronephros of Dipnoi alone is wholly unknown. Beard ('90, 

 p. 157) speaks of the transformation of a part of the pronephros into the 

 Miillerian duct as "a well known fact"; but the only authority he cites 

 in this connection (Parker, '89) does not make such a statement, nor 

 have I succeeded in finding anywhere in the literature any account of the 

 pronephros of Dipnoi. Unless Beard has personal observations on this 

 matter, I believe that in Dipnoi absolutely nothing is known of the 

 pronephros or its transformation, save such inferences as may be drawn 

 from the adult anatomy. I shall therefore merely repeat the statement 

 of Ayers ('S5, p. 506), that the development probably proceeds as in 

 Amphibia, since the adult urogenital system in this group presents the 

 closest analogy with that of the Dipnoi. 



The excretory system of Cyclostomes is similar to that of Amphibia. 

 In Petromyzon a pronephros develops in the Ammocoetes larva, but 

 aborts in the adult. The number of nephrostomes and of tubules is 

 small (4, according to Willi. Muller; 4 to 5, Shipley; 3, Kupffer; 

 according to Semon, an inner and an outer row of nephrostomes are 

 to be distinguished); and they communicate with an anterior expanded 

 portion of the body cavity. According to Fiirbringer ('78 a , p. 42), the 

 pronephros extends over about four somites. Opposite the nephro- 

 stomes, a vascular organ projects from the root of the mesentery into 

 the body cavity. This is the so-called glomerulus; as figured by Scott 

 ('81, Taf. IX. Fig. 24), it strikingly resembles the glomus of Am- 

 phibia. According to Scott, the pronephric tubules develop secondarily 

 as outgrowths from the segmental duct. On the other hand, Shipley has 

 confirmed the statements of Muller and Fiirbringer, according to which 

 the nephrostomes and tubules are formed by the incomplete closure 

 of a longitudinal groove of somatopleure. Finally, Kupffer maintains 

 that the tubules arise as three separate evaginations of the somatopleure, 

 a result which is in harmony with my own observations on Amphibia. 1 



In Myxine nothing is known of the early development ; but in late 

 stages an organ has been made known by the studies of "Willi. Muller 



1 In Goette's ('88, p. 163) preliminary account of the development of Petromyzon 

 he states that a pronephros develops in the same manner as in Amphibia. This 

 would indeed be a conclusion acceptable to me, but until the accounts are more at 

 one in regard to the latter group the statement is somewhat vague. I await with 

 interest the publication of that portion of Goette's final paper which relates to the 

 excretory sj'stem. 



