640 BASHFORD DEAN, 



of their development : and indeed even within their sub-groups there 

 occur such striking ditferences in outogeny tliat a morphologist 

 niay well wonder to what degree may not adaptations be impressed 

 upon the structures of the embryo. Within a Single family like 

 that of the Cyprinidae developmental differences may be found which 

 according to older tenets must certainly be interpreted as of far 

 greater significance than those existing between the classes of Reptiles, 

 Birds and Mammals. 



It has been noticed that differences in the adaptive characters 

 of the different groups of fishes sometimes appear most strikingly in 

 the stages of latest and earliest development. In the previous papers 

 the writer has explained some of relationships of the most pro- 

 nounced features of the early development of Ganoids; at present he 

 would undertake to show that the later embryonic and the larval 

 development of the most modern type of Ganoid demonstrates clearly 

 marked transitions to the larval couditions of Teleosts, and that 

 therefore no broad line can be drawn between the typical conditions 

 in the Teleost and those of the most archaic form, Lepidosteus, as 

 described by A. Agassiz. 



The larval conditions of Ganoids it should be said in introduction 

 are remarkably uniform during the first day after hatching in all 

 forms examined; the larvae of Lepidosteus^ Acipenser and Ämia at 

 this stage would not readily be distinguished were it not for the well 

 marked diöereuce in their size. In all forms the yolk sac is of the 

 same relative form, and the degree of the diflPerentiation of sense 

 Organs, trunk. fins, viscera, surprisingly uniform. But from this time 

 onward the dififereiitiation of the larvae takes place rapidly ; on the 

 second day the diti'erent forms have become dissimilar; and during 

 the next two days the conditions already suggest those of the niature 

 fish, of Ämia ou the third day, of Acipenser on the fourth and of 

 Lepidosteus on the fifth to sixth. In traciug these developmental 

 advances we cannot fail to be impressed witli the greater precoci- 

 ousness ^) of Ämia in coutrast with the other forms, a distinct near- 

 ing accordingly to the Teleostean. 



The following anangement has been made in describing the larval 

 development of Ämia: 



1) The differences in the rate of development during the first 

 days of development could not have been noteworthy ; the larvae of all 

 forms were hatched out and reared in rooms which could have varied 

 but little in point of temperature. 



