FERTILISATION 205 



protoplasm may be due to surface-tension forces, and that the 

 conditions for this process may depend upon the surface tension 

 between the spermatozoon and the sea-water becoming greater 

 than the sum of the surface tensions between the sea-water and 

 the egg, and the spermatozoon and the egg. Loeb remarks, 

 further, that the fertilisation of Strongylocentrotus eggs by 

 sperms of the same species can best be accomphshed in normal 

 sea-water, and with this observation he associates the fact that 

 the mobility of the Strongylocentrotus sperms is diminished by 

 the alkaline water.^ 



While suggesting that restrictions to the power of cross- 

 fertihsation may be due to differences in surface tension, Loeb 

 admits that the evidence seems to show that the capacity to 

 conjugate is to some extent at least specific. Attempts were 

 made to fertihse the eggs of sea-urchins with the spermatozoa of 

 Annelids and Molluscs, but these experiments were without 

 success. Very recently, however, Kupelweiser ^ reports that 

 he has been successful in fertilising Strongylocentrotus ova 

 with the spermatozoa of the mussel (Mytilus), and that the 

 products developed into gastrulae. 



Dr. A. T. Masterman tells me that, in certain cases, hybridisa- 

 tion among fishes may be induced more readily in the absence 

 of opportunity for normal fertilisation, that is to say, for fertilisa- 

 tion of ova by spermatozoa of the same species. If such ova 

 are present, the spermatozoa tend to conjugate with them rather 

 than with ova belonging to a different but closely aUied species. 

 It would appear, therefore, that the spermatozoa exhibit an 

 elective affinity for ova belonging to the same species as them- 

 selves. This has been shown especially in hybridisation experi- 

 ments between brill and turbot.^ 



That assortative mating amongst gametes occurs generally as 

 the result of a preferential tendency possessed by them towards 

 conjugating with other gametes bearing similar characters to 



' Loeb, The Dynamics of Living Matter, New York, 1906. 



' Kupelweiser, "Versuche iiber Entwiokelungserregung und Membran- 

 bildung bei Seeigeleiern durch MoUusksperma," Biol. Centrabl., vol. xxyi., 

 1906. 



' M'Intosh and Masterman, Life History and Development of the Food 

 Fishes, and articles in the Reports of the Scottish Fishery Boards, 9th Rep., 

 Pt. III., 10th Rep., Pt. III., and 13th Rep., pt. III. 



