HYBRIDS BETWEEN FUNDULUS AND MACKEREL 415 



results nearly uniformly as indicated in the typical experiment 

 given in detail. Later in the season the results differ materially. 

 In this paper our attention is confined to the results obtained in a 

 number of crosses made about June 12 to 15. 



8. The general conclusion derived from a study of the data are: 

 in proportion as the paternal element vigorously exercises its 

 functions, in like proportion is development retarded and the 

 various types of monster appear. Likewise the paternal types 

 of chromatophore tend to be more numerous the more pronounced 

 the general abnormality. The most successful embryos are, in 

 so far as the chromatophores are concerned, pure maternal, a 

 fact that leads to the belief that these practically normal indi- 

 viduals are not the product of parthenogenesis, but represent 

 cases of complete recovery from an influence that in most cases 

 retards development. It seems that the disharmonious elements 

 of the paternal contribution are either completely neutralized 

 or so modified as not to oppose the normal activities of the 

 maternal materials. 



9. The great majority of hybrids are subnormal, especially in 

 their apical structures (eyes, hearts, etc.), and at the same 

 time show obvious paternal heredity. The persistence of pater- 

 nal types of chromatophores indicates that the paternal contribu- 

 tion is still active and that complete recovery is therefore impos- 

 sible. The embryos are largely to be classed as the results of 

 permanent but nonlethal inhibiting agents, the effect of which is 

 always, according to Child's ^ axial gradient' hypothesis, to pro- 

 duce embryos with more abnormalities in the apical than in the 

 basal parts. Hence we explain the wide range of ophthalmic 

 and cardiac abnormahties in these hybrid groups as the result 

 of differential inhibition. 



10. A somewhat smaller group of hybrid embryos show large 

 apical parts and reduced basal parts. Occasionally isolated 

 apical parts (eyes and hearts) occur in an otherwise undifferenti- 

 ated egg. Such embryos are usually pure maternal as to chro- 

 matophores or at least in the region of differentiation. Such 

 anomalies are to be interpreted as differential recovery products, 

 but differ from the complete recovery cases in that recovery 



