MUSEUM OF COMPARATIVE ZOOLOGY. 305 



Van Wijhe ('8S a , '89), Ruckert ('88), Hoffmann ('89), and Wieder- 

 sheim ('90), have distinctly denied the serial homology of the pronephros 

 and the mesonephros. The objections of these authors to the view which 

 I have adopted have been most clearly formulated by van Wijhe ('89, 

 pp. 509, 510), whose account I shall follow in my criticism of their 

 position. First, "the pronephros arises before the appearance of the 

 duct or the mesonephros, and is indeed the first part of the excretory 

 system that appears." This point of difference is, as I have stated, the 

 most conspicuous feature in which the two glands are unlike. It is, how- 

 ever, not a weighty argument against their serial homology. Secondly, 

 " the pronephros arises as an (in Selachii segmented) evagination of the 

 somatopleure ; its cavity, which may be temporarily obliterated by the 

 proliferation of the walls, is formed as an evagination of the body cavity 

 (Metacolom). The mesonephros, on the other hand, is not formed as an 

 evagination, and it is constituted of somatopleure as well as of splanchno- 

 pleure." This analysis seems at first sight to establish a fundamental 

 contrast between the pronephros and the mesonephros, and I admit fully 

 the cogency of the argument in disproving a comparison of the nephro- 

 stornal and glomerular portions of a mesonephric tubule with the 

 nephrostomal canal of the pronephros. On the other hand, however, I 

 would insist that a hitherto unnoticed homologue of the pronephric evagi- 

 nations is to be found in the outward growth of the primitive mesonephric 

 canal to join the duct. 1 (See page 301.) It is in precisely this way that 

 a tendency to a somatopleural evagination would of necessity manifest 

 itself. Thirdly, "the duct always appears in continuity with the pro- 

 nephros, but always discontinuous with the mesonephros, which only 

 secondarily fuses with it and empties into it." This circumstance, as I 

 have already shown, is a direct consequence of the condition explained 

 under the first head. Fourthly, "the mesonephros possesses Malpighian 

 corpuscles ; while the pronephros has none, the glomus of the latter not 

 being homodynamous with the glomeruli of the mesonephros because it 

 is a vascular tuft invaginated into the secondary body cavity (Meta- 

 colom)." This contrast appears to me morphologically inaccurate, as I 

 believe I have adequately shown in the preceding discussion. 



A further objection, which van Wijhe does not mention in his enumera- 

 tion, is the occurrence of rudimentary mesonephric tubules in the somites 

 which formerly gave rise to the pronephros. To prove this assertion, it 



1 This is the only portion of the mesonephric tubule which can properly be called 

 an evagination; the entire tubule comprises the evagination plus the communicat- 

 ing canal. 



vol. xxi. — NO. 5. 20 



