212 THE PHYSIOLOGY OF REPRODUCTION 



with regard to the skeleton, which has been used so extensively by 

 previous workers in this subject. In the late larval life, characters 

 such as the posterior ciliated epaulettes or ciliated rings are ones 

 that show very little variation and to which no exception can be 

 taken. E. esculentus and E. acutus always develop the posterior 

 ciliated epaulettes while E. miliaris never possess these structures. 

 In crosses between these forms conducted over three years, maternal 

 inheritance with regard to this character was invariably observed, 

 the reciprocals of a cross being unlike. In the fourth year's work, 

 however, the inheritance of this character proved different and gave 

 a dominance of E. esculentus and E. acutus over E. miliaris, both 

 reciprocals of a cross being alike. No reason for this change could 

 be suggested. The cytological study of material of all the crosses 

 during the course of the work showed that in all instances true 

 fusion of the pronuclei during fertilisation took place, but varying 

 amounts of chromatin were thrown out of the nuclei in different 

 crosses and in different experiments. 



Loeb l discovered that cross-fertilisation of the eggs of Strojujylo- 

 centrotus 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 Strongylo- 

 centrotus 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-protoplasm 

 may be due to surface-tension forces, and that the conditions for this 

 process may depend upon the surface tension between the sperma- 

 tozoon 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 accomplished in normal sea-water, and with this observa- 

 tion he associates the fact-that the mobility of the Strongylocentrotus 

 sperms is diminished by the alkaline water. 2 



1 Loeb (J.), " Ueber die Befruchtung von Seeigeleiern durch Seesternsamen," 

 Pftiigei^s Archiv, vol. xcix., 1903. " Weitere Versuche iiber heterogene 

 Hybridisation bei Echinodermen," Pfltige^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. 



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



