414 H. H. NEWMAN 



tions and are considered as mechanical mixtures. Real hybrid 

 phenomena appear to be restricted to species within the order 

 Diademoida of the class Echinoidea. 



2. In fish hybridization the same situation appears. The 

 hybrids known for fish are restricted within the confines of the 

 order Teleostei. 



3. Subordinal crosses in both Echinoids and Teleosts have been 

 made, but the data for crosses of this width among Echinoids is 

 contradictory and very unsatisfactory; hence the especial value 

 of an intensive study of Teleost hybrids of this width of cross. 

 Practically all Teleosts may be crossed without artificial chemical 

 aids. 



4. As an example of a heterogenic Teleost hybridization of 

 suborder width, that between Fundulus heteroclitus and Scom- 

 ber scombrus is chosen after an extensive survey of a wide range 

 of crosses, because it is especially well adapted to show the facts 

 about heterogenic hybridization in a clean-cut and unequivocal 

 way. Other crosses nearly as good might have been chosen. 



5. Although the adult characters of the two species are in 

 striking contrast and the larvae at all stages are different in all 

 details, the only reliable differentiating characters of the younger 

 larval stages are the chromatophores. Fundulus has red chromat- 

 ophores and the mackerel green ones; Fundulus has solid black 

 chromatophores and the mackerel delicately branched black ones ; 

 these differentiating characters serve to indicate the degree to 

 which the two parental elements function in development and 

 heredity. 



6. Cross fertilizations between the two species result differently 

 according to which is used for the egg species. When Fundulus 

 eggs are used, all grades of success in development up to hatched 

 larvae are obtained. When mackerel eggs are used, all embryos 

 die before or during gastrulation, before any specific characters 

 are differentiated. The study of heredity is therefore confined 

 to one of the two reciprocal crosses, that between Fundulus 9 

 and Scomber cf . 



7. If the experiment is performed near the middle of June, 

 when the breeding season of both species is at its height, one gets 



