The Process of Heredity 505 



physiological resistances, etc. Only to this extent then is the 

 developmental history of the bony fish treated, hence no apology 

 is offered for the incompleteness of embryological detail. 



The experiments of Boveri, Driesch, Herbst, Seeliger, Fischel, 

 etc., have for this work only a casual significance for the reason that 

 the prime interest of the authors was focused upon another prob- 

 lem, viz: the comparative potency of nucleus and cytoplasm in 

 heredity. No attention was paid to heredity as a process and 

 this is the chief idea brought out in the present work. No formal 

 review of this rather voluminous literature on Echinoderm hybrids 

 is attempted in this place. Only where the facts presented have 

 a distinct bearing on the work in hand will the data of these 

 authors be referred to.^ 



Probably no type of egg offers so many advantages for the 

 study of heredity as a process as that of the bony fish, and it is 

 equally probable that few species of fish are so available for 

 this kind of work as those used in the breeding experiments here 

 described. Some of the most obvious advantages of the species 

 used are as follows: 



1 Both species are abundant and easily obtained. 



2 Both species thrive well under laboratory conditions. 



3 The adults are of convenient size, neither too large nor too 

 small, and yield large numbers of eggs that are easily stripped. 



4 The spawning behavior and sexual dimorphism of both 

 species were easily studied and a knowledge of these phenomena 

 proved to be a prerequisite for the present study. 



5 The adults possess a sufficiently large number of differenti- 

 ating characters to furnish data for the comparative study of adult 

 hybrids, should it prove feasible to rear the young fish to maturity. 



6 The eggs of both species are of convenient size, and, what is 

 of more importance, are of quite unequal size. 



7 The eggs are nearly transparent and the development of all 

 characters can be studied without difficulty in living material. 



8 The embryos can be studied for long periods as stationary 



^ For a comprehensive review of recent work on Echinoderm hybrids, see Alfred Fischel's paper, 

 "Ueber Bastardierungsversuche bei Echinodermen.'' Arch. f. Entw.-Mech, 22, 1906. 



