996 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [16J 



blastic blastema may have been laid down in tbe primitive terminal 

 somites, so that the urosome was slender and weak from the em- 

 bryonic period onward. This might readily lead to the degeneration of 

 the caudal fin -fold, for the reason that no surplus of mesoblast was pre- 

 viously laid down in the viciuity which could be proliferated into the 

 termiual fin-folds, conseqnently no caudal rays, apophyses, or even fin- 

 folds were developed. Degeneration would then assert itself in the 

 higher forms by the development of a transient opisthure ; in the lower 

 forms either a i)ersistent opisthure would be formed, or the Avhole tail 

 would become slender, weak, and flagelliform, as in the Rays and in 

 Oastrostomus. 



With the loss of mobility, or rather of muscular power, owing to a 

 lack of well-defined muscular segments, it would result that no verte- 

 bral segments or only very imperfectly differentiated vertebrae would be 

 formed, such as we actually find to be the case in Gastrostomus. Con- 

 verse leasoning from another set of facts leads to a similar conclusion. 

 In every case where a well-marked urostyleis developed from the point 

 of axial flexure it is unsegmented w^heu fully formed, as in Amiurus, or 

 it is unsegmented at its posterior extremity only. The degenerate pos- 

 terior extremity of the chorda often becomes covered by a continuous 

 osseous investment which renders it inflexible, a condition which could 

 not have been established if the musculature on either side had been 

 well enough developed, and the end of the urosome had retained its 

 archaic or straight form. When, however, heterocercy was developed 

 the musculature of the tail became subordinated to a new function, viz, 

 flexing the powerful caudal rays upon the hypural bones ; the forces 

 competent to induce segmentation of the urostyle are therefore absent, 

 as it is usually found that the segmentation of hard parts corresponds 

 pretty closely in tlie lower forms to the points of segmentation or to the 

 points of origin and insertion of the muscles. With the advance in the 

 differentiation of the muscles, as in the limbs of mammalia for example, 

 this correspondence is less marked, being obscured by secondary adap- 

 tation. 



The evidence in favor of the doctrine that many of the recent Tele- 

 osts, as well as the recent and fossil Ganoids, have descended from an 

 ancestry the urosome of which was more prolonged or contained more 

 segments than the urosome of existing species, is therefore quite con- 

 clusive, and that what has led to this decrease has been the further 

 functional specialization of the caudal fin. In fact, a survey of the 

 Vertebrates, taken as a class, leads to the conclusion that in almost all 

 orders there has been a tendency in the course of their i^hyletic histo- 

 ries towards a degeneration of the caudal part of the axial skeleton. 



The remarkably uniform tendency of nearly all of the Lyrifera to be- 

 come heterocercal, involving the degeneration of certain parts which 

 enter into the tail, would indicate that similar causes were operative 

 amongst such a diversity of forms, jn'oductive of such similar morpho- 



