OF ARTS AND SCIENCES. 271 



XVI. 



ON THE YOUNG STAGES OF SOME OSSEOUS FISHES. 

 By Alexander Agassiz. 



Presented May 9, 1882. 



Part HI.* 



Many interesting points of relationship between the embryos of 

 osseous Fishes and their fossil representatives have been traced by. 

 comparing the structure of the tail of the fish embryo as it passes 

 from the leptocardial stage through the various stages of heterocer- 

 cality to a so-called homocercal stage. This relationship, as has been 

 pointed out, is very marked, and has led to some important generaliza- 

 tions. The comparison of the pectorals or of the dorsal and anal 

 fins does not, however, lead to such interesting results. It is true that 

 as far as the pectoral fins are concerned, their resemblance in the 

 early stages of the bony fish embryo to the crossopterygian type of 

 pectorals is very striking, but, owing to our imperfect knowledge of 

 the structure of the pectorals of the ancient Fishes, this comparison is 

 at present less complete than that between the tails of the older fossil 

 Fishes and the tails of the embryos of the modern osseous Fishes. 



With regard to the comparison of the median fins of the osseous 

 Fishes of to-day with the median fins of Fishes of earlier periods, we 

 do not come to any satisfactory results. If we take, for instance, the 

 change undergone by the embryos of osseous Fishes, we find invaria- 

 bly in the youngest stages a continuous embryonic fold, extending from 

 the head along the dorsal side to the extremity of the tail and around 

 the lower side to the yolk bag. At a later period, when they carry 

 embryonic rays, these embryonic median fins resemble somewhat the 



* Part I. Proc. Amcr. Acad. XIII. 1877-78, p. 117 ; Part II. Proc. Amer. 

 Acad. XIV. 1878-79, p. 1. 



