450 Prof. J. B. Farmer. On the [May 13, 



Halidrys, I have been able, though not so easily, to detect the repellant effect 

 in some other Fucaceae. Still more recently, in an investigation on ferns 

 involving the details of fertilisation, we have observed that after one 

 sperm had entered the egg, the others close to its surface showed signs of 

 greater disorganisation than those which were further removed from it. In 

 this instance also, then, it seems clear that the entrance of a sperm produces 

 a chemical change in the egg which is reflected over its whole surface, although 

 we are not as yet in a position to make any definite statement as to the 

 precise nature of the substance concerned, 



It is another question whether the 3hange is produced directly in the 

 cytoplasm, or whether it emanates in the first place from the nucleus. 

 Although certain facts seem at first sight to favour the latter view, I think 

 the former is the more correct one. In many gymnosperms the two sperms 

 which are formed in the pollen tube are both extruded into the egg. But 

 only one of them is functional; the other soon disintegrates in the cyto- 

 plasm, and does not approach the egg-nucleus. But it must be borne in 

 mind that the sperms are rather passively shot into the egg from the pollen- 

 tube, than individually attracted into it. In Cycads, however, where the 

 sperms are actively motile, only one enters the egg as a rule, and when more 

 do succeed in penetrating it, they always perish at once. It may be 

 suggested that as the egg of these plants is of enormous size as compared 

 with that of the Fucacese, the change which is produced instantaneously 

 in the latter may take longer to complete in the case of the gymnosperms. 

 In any event it does take place, and the destruction of the additional sperms 

 occurs in time to prevent them getting near the nucleus of the egg. 

 Further evidence in favour of the cytoplasmic origin of the repellant 

 substance is again afforded by Halidrys. It often happens that the eggs, as 

 they are being shed, are broken into two pieces, of which one is destitute of 

 a nucleus. Such fragments, however, as Williams has shown, commonly* 

 attract the sperms, and on the entrance of one of them the same repellant 

 influence on the rest is as evident as in the case of normal eggs. Wilson, 

 also, found conclusive evidence to show that when enucleated fragments 

 of echinoderm-eggs are fertilised, only a single sperm is concerned in the 

 process. 



These observations seem to me to prove that the essential act of fertilisa- 

 tion consists not only in the union of two, and not more than two, nuclei, 

 but also in the retention by them of a definite structural basis. For we have 

 seen that the mere addition of nuclear material cannot be regarded as the 

 important act, inasmuch as any extra sperms entering the egg are without 

 influence on development, prejudicial or otherwise, provided that, as in the 



