72 WILLIAM MORTON WHEELER, 



believe that the mesonephric tubules are serial homologues of the 

 pronephric tubules. 



In the above cited cases of substitution, both in Invertebrates and 

 Chordata, the new systems that replace the old obviously arise in 

 response to a need for more excreting surface — a need, which, 

 during the phylogeny, may have acted as the same stimulus that still 

 appears to call forth additional tubules during the ontogeny, viz. the 

 growth of the organism in size. The excreting surface increases in a 

 kind of geometric proportion like the increase in the bulk of the 

 animal. In Vertebrates this is brought about, as Habl (1896) has 

 shown, by a decrease in the calibre together with an increase in the 

 number and length of the tubules substituted in each successive 

 generation. Thus the tubules of the mesonephros are more numerous, 

 of smaller calibre, and longer than those of the pronephros, and the 

 tubules of the metanephros exhibit in turn a still higher degree of 

 development along the same lines. We may, perhaps, explain the 

 peculiar multiplicity of funnels in the pronephric tubules of Amphioxus 

 and Myxinoids as brought about by a need of greater excreting 

 surface in these animals before the mesonephros made its appearance 

 in the phylogeny as a better adaptation to the same end. In the 

 embryo of Amphioxus and Myxinoids the nephridia are very probably 

 simple tubules with a single opening like those at the extreme anterior 

 and posterior ends of the pronephric series in the adult of the former 

 animal. The subsequent development of several nephrostomes with a 

 concomitant branching of the tubule are sufficient to supply the need 

 for more excreting surface in a small Chordate like Amphioxus. In 

 Myxine the same method of supplying the need was first adopted but, 

 proving insufficient, an adventitious series of nephridia, the mesonephros, 

 was called into existence. The mesonephridia , however, remained 

 small and straight, while the pronephric duct, made up of confluent 

 pronephric tubules of the primitive simple pattern, continued to function 

 as of old. In Petromyson the pronephric tubules have probably retained 

 the single original funnel, and, in response to the need for more ex- 

 creting surface, have simply lengthened and become convoluted instead 

 of ramifying, while the mesonephros has taken on a much more com- 

 plicated structure, developing piecemeal with the growth of the animal 

 in the form of long, convoluted and slender tubules, far outnumbering 

 the metameres. In all Gnathostomes the pronephric tubules have 

 the simple primitive nephridial structure. They have either lost or 

 have never developed multiple funnels. 



