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



the nucleus escapes from its cytoplasmic investment which then disintegrates 

 and mixes with that of the egg, whilst the two nuclei approximate and often 

 fuse. Although the amount of cytoplasm thus introduced is exceedingly 

 minute, as compared with that of the egg, probably no one at the 'present 

 time would regard this circumstance as necessarily implying a correspondingly 

 small degree of importance, unless other evidence pointed also to the same 

 conclusion. 



Boveri has shown that when an enucleated fragment of the egg of one 

 species of echinoderm is fertilised by the sperm of another species, or even 

 genus, the resulting larva resembles the male parent. These statements 

 have been sometimes challenged, but the explanation of the discrepant 

 results obtained by different observers probably lies in the fact that the eggs 

 themselves may be readily stimulated to develop parthenogenetically. 

 Enucleated fragments of eggs already in this condition, even in the earliest 

 stages, will have already received a developmental stimulus of a material 

 nature from their own (i.e., the female) nucleus. This must inevitably 

 produce an effect on the final result. The recent discovery* that if eggs which 

 are known to be in the early stages of parthenogenetic development are 

 fertilised by the sperm of another species, the offspring actually do show 

 a markedly increased resemblance to the maternal parent, is of the highest 

 theoretical interest in connection with the mode of interaction between the 

 nucleus and the cytoplasm, . 



The entrance of more than one sperm, and the fusion of their nuclei with 

 that of the egg, is commonly followed by great disturbance in the course of 

 development of the embryo, if, indeed, it is able to proceed at all. It is 

 remarkable how very seldom polyspermy occurs in healthy cultures. I have 

 myself examined many thousands of the eggs of Fucus, and I have only seen 

 two or three examples of the entrance of more than one sperm into each egg. 

 In some cases, however, polyspermy can be readily induced by anaesthetising, 

 or in other ways interfering with the normal conditions of the egg ; and the 

 abnormal results that ensue serve to emphasise the correlation existing between 

 nuclear constitution, and cell- and tissue-development. 



The rarity of polyspermy, at any rate amongst the majority of plants, points 

 to the existence of some means of actively preventing its occurrence in 

 ordinary eggs. It is often stated that a cell wall is instantly excreted over 

 the surface of the egg on fertilisation, and that in this way the entrance of 

 additional sperms is prohibited. Such an explanation, if, indeed, it ever 

 suffices, will not cover every case. In some eggs, as, for example, those of 

 the Fucacese, the membrane is not formed till an appreciable time has elapsed 



* Curt Herbst, " Vererbungsstudien, IV," ' Arch. f. Entw.-Mech. d. Organismen,' vol. 22. 



