HYBRIDS BETWEEN FUNDULUS AND MACKEREL 413 



understandings. We shall all then be able to admit that these 

 'mechanical mixtures/ which some authors have called 'hetero- 

 geneous' or heterogenic hybrids, are pure maternal and may 

 be parthenogenetic as to the initiation of development. On 

 the other hand, we must expect to find in true hybrids (i.e., 

 all those in which the male germ cell shows morphological evi- 

 dence of playing a role in development) an accompanying physio- 

 logical effect upon development and heredity. Personally I 

 feel that a morphological picture must represent an equivalent 

 result of functioning and must presage further functioning. It 

 may well be that the egg develops more normally when the 

 activities of the foreign sperm are suppressed, as is evidenced by 

 nearly normal and pure maternal larvae among Teleost hybrids, 

 and that marked activity on the part of the sperm material 

 greatly retards and inhibits normal development ; but this is onl}^ 

 what we should expect in true hybrid combinations of consid- 

 erable heterogeneit}^ In crosses between closely related species, 

 however, the effect of only slightly diverse materials appears to 

 act as a stimulus, and more rapid development and more vigor- 

 ous offspring frequently result. It is interesting to know that 

 there is every degree of gradation between the two extreme 

 results in true hybridizations. In some extreme heterogenic 

 crosses the functioning of the sperm is so vigorous as to retard 

 normal development during cleavage so seriously that differenti- 

 ation is wholly inhibited. At the other extreme are those cases 

 in which even during cleavage the processes of development are 

 accelerated and supernormal individuals result. All of these 

 results are rightly to be included within the category of hybrid- 

 ization phenomena, 



SUMMARY 



1. A survey of the literature on Echinoid hybridization is 

 attempted in order to gain an accurate idea of the relative avail- 

 abiUty of Echinoids and Teleosts for the study of hybridization 

 phenomena. It appears that by chemical means inseminations 

 of Echinoid eggs with sperm of other orders, classes, and even 

 phyla may be made, but these involve no real fertilization reac- 



