994 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [14] 



itive worm-like plau recapitulated by einbrj'ouic development, leads to 

 the differentiation of the anterior or cephalic and somatic parts of the 

 form from the caudal part. Such a differentiation really leads to the 

 production of a number of very important morphological and physio- 

 logical chaugeSj but it also at the same time affords embryological evi- 

 dence of the fact that the theory of serial homologies founded on the tail 

 of the eel is correct. 



The degeneration — possibly specialization — which we have observed 

 to occur in the course of the development of the tail of embryos is fol- 

 lowed by other degenerative processes of considerable significance. The 

 most important of these are, of course, those which lead to the evolution 

 of heterocercy and the formation of a urostyle,but the instances in which 

 we observe degeneracy to be so palpable that we cannot deny the fact of 

 its existence are api)arently those of Chimcera monstrosa, Gastrostomus 

 Bairdii, and ^tijlejjhorus chonJatus, in which a long opisthiirc has been 

 formed, which at a very early period was vermiform and practically 

 without developed median tin-folds. This almost useless opisthure re- 

 sults from the failure of the animals under consideration to develop in 

 this region, during their early stages, well-marked myotomes, or if they 

 ever did develop they subsequently degenerated more or less com- 

 pletely, leaving little or nothing behind except the chorda invested by 

 connective tissue and integument. 



If, as it is possible to conceive, the original form of the vertebrate 

 body had more segments than it now has in even such comparatively 

 simple forms as heterocercal Teleosts, it is not unreasonable to suppose 

 that the failure to fully develop the terminal myotomes actually helped 

 to lead to the initiation of heterocercy, for the reason that not sufficient 

 material was built up into muscular segments at the tip of the tail tt> 

 supply the muscular bundles for the formation of the flexors and di- 

 varicators of the caudal rays of heterocercal firms, the musculature 

 of which has been really derived by differentiation of certain antepe- 

 nultimate myogenous segments. This failure of the myogenous tract 

 to develop in the last segments may have resulted partially from the 

 sufficiency of the more anterior part of the urosome for purposes of pro- 

 pulsion in a watery medium, so that its posterior ])art failed to become 

 functional from disuse, and was from such a cause atrophied, owing to 

 a diminishing blood-supply which must have supervened. 



I am also aware that the number of myotomes found in Teleost em- 

 bryos before the body becomes free anteriorly and posteriorly from the 

 yelk varies in different families, so that the preceding argument must 

 be qualified by just so much as such variations in the number of larval 

 somites developed may be supposed to affect the number in the adults 

 of different groups. This larval variation actually depends upon the 

 number of segments functionally developed in the adult, as is shown 

 by the development of Belone [Tylosurns], by Fierasfer, and the Lepto- 

 cephalid stages of certain marine eels, when contrasted with the young 



