204 THE PHYSIOLOGY OF REPRODUCTION 



mented with forty-nine different combinations, obtained results 

 which 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, Strong ylocenlrotus, were supposed to grow more 

 and more " prepotent " as they became more and more mature. 

 Doncaster, 1 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. 



Loeb 2 discovered that cress-fertilisation of the eggs of 

 Slronyylocenlrotus by the spermatozoa of various species 

 of starfish could be effected by adding sodium carbonate or 

 sodium hydroxide to the sea-water in just sufficient quantity to 

 render it slightly alkaline. Under these conditions as many as 

 fifty per cent, of the Strong ylocentrotus eggs could be fertilised 

 by Asterias spermatozoa, whereas in normal sea-water cross- 

 fertilisation between these two Echinoderms only occurs very 

 exceptionally. What the nature of the change is whereby the 

 alkaline sea-water enables the sperm to fertilise the ova does 

 not appear to be known. It has been observed that the addition 

 of the alkali increases the motive-power of the sperms, but the 

 same result is brought about by bicarbonate of sodium, without 

 augmenting their capacity to cross-fertilise. Loeb suggests that 

 the entrance of the spermatozoon into the interior of the egg- 



1 Doncaster, "Experiments in Hybridisation," Phil. Trans. B., vol. cxcvi., 

 1903. MacBride ("Some Points in the Development of Ophiothrix fruyilis," 

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

 (ovarian) ova of the Ophiuroid, Ophiothrix, 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. 



2 Loeb, " Ueber die Befruchtung von Seeigeleiern durch Seesternsamen," 

 Pjluger's Archiv, vol. xcix., 1903. " Weitere Versuche iiber heterogene 

 Hybridisation bei Echinodermen," Pjluger's Archiv, vol. civ., 1904. See 

 also translation of the latter, as well as other papers, in the University of 

 California Publications, Physiology, vols. i. and ii., 1902-4. 



