FERTILISATION 211 



Among the Mammalia, as is well known, cross-fertilisation 

 between nearly allied species commonly occurs. The resulting 

 hybrid may be either sterile {e.g. the mule) or fertile {e.g. the hybrid 

 offspring of the bull and American bison). There is no evidence 

 that more widely separated species of Mammals can be induced to • 

 have hybrid offspring. Spallanzani,^ by artificially inseminating an 

 oestrous bitch with the spermatozoa of a cat, attempted such an 

 experiment, but without a positive result. 



A number of investigators have effected cross-fertilisation between 

 various kinds of Echinoderms. Vernon,- who experimented with 

 forty-nine different combinations, obtained results whick were more 

 or less successful in thirty-seven. In some of these, however, 

 development did not proceed beyond the blastula stage. Vernon 

 attempted to show that the capacity of the animal to transmit its 

 characters to its hybrid offspring depended upon the condition of 

 ripeness or staleness of its gametes at the time of fertilisation. 

 Thus, the spermatozoa of the sea-urchin, StroTvgylocentrotus, were 

 supposed to grow more and more " prepotent " as they became more 

 and more mature. Doncaster,^ however, has described further 

 experiments which seem to indicate that the variation in the form of 

 the hybrids obtained by Vernon was really due to differences in the 

 temperature of the water. 



Shearer, de Morgan, and Fuchs * have been able to obtain an Fg® 

 generation from the cross Echinus esculentus ^ x U. Mutus$ at 

 Plymouth, but were unable to obtain fertile individuals of the cross 

 E.miliaris^ X E.esculentusS or E.miliaris'} x E. acukis^ although 

 fairly large and healthy Fj individuals of both these crosses were 

 reared. In all instances the gonads of the ¥^ crosses with E. miliaris 

 failed to develop beyond a very rudimentary condition. As the 

 result of extensive investigation of the earfy larval characters, these 

 authors come to the conclusion that they are too variable to afford any 

 trustworthy evidence of parental Influence. This is particularly so 



' Spallanzani, Dissertations, English Translation, vol. ii., London, 1784. 



^ Vernon, "The Relation between the Hybrid and Parent Forms of 

 Echinoid Larvae," Phil. Trans., B., vol. cxc, 1898. 



3 Doncaster, " Experiments in Hybridisation," Fhil. Trans., B., vol. cxcvi., 

 1903. MacBride ("Some Points in the Development of Ophiothnx fragilis," 

 Proc. Roy. Soc, B., vol. Ixxix., 1907) has recently shown that the immature 

 (ovarian) ova of the Opliiuroid, Ophiothi-ix, may be fertilised, but that the 

 subsequent development is abnormal, segmentation resulting in a morula 

 instead of a blastula, while at the stage at which the archenteron is formed, 

 there is a tongue of cells projecting into its lumen. It appears, therefore, that 

 the stage of maturity at which ova are fertilised may affect their embryonic 

 development if not their hereditary characteristics. 



* Shearer, de Morgan, and Fuchs, "On the Experimental Hybridisation 

 of Echinoids," Phil. Trans. Roy. Soc. Land., Ser. B., vol. cciv., 1914. See 

 also H. M. Fuchs, Jour. Mar. Biol. Ass., vol. x., 1914. 



° See footnote, p. 202. 



