IV] AND STRUCTURE OF THE CELL 335 



greater and greater histological skill are not the one and only way 

 to study the physical forces acting within the cell *. 



As regards the phenomena of fertihsation, of the union of the 

 spermatozoon with the "pronucleus" of the egg, we might study 

 these also in illustration, up to a certain point, of the forces which 

 are more or less manifestly at work. But we shall merely take, as 

 a single illustration, the paths of the male and female pronuclei, as 

 they travel to their ultimate meeting-place. 



The spermatozoon, when within a very short distance of the egg- 

 cell, is attracted by it, the same attraction being further manifested 

 in a small conical uprising of the surface of the eggt. The nature 

 of the attractive force has been much disputed. Loeb found the 

 spermatozoon to be equally attracted by other substances, even by 

 a bead of glass. It has been held also that the attraction is 

 chemotropic, some substance being secreted by the egg which drew 

 the sperm towards it: just as Pfeifer, having shewn that maUc acid 

 has an attraction for fern-antheridia, supposed this substance to 

 play its attractive part within the mucus of the archegonia. Again, 

 the chemical secretion may be neither attractive nor directive, but 

 yet play a useful part in activating the spermatozoa. However 

 that may be, Gray has shewn reason to believe that an electromotive 

 force is developed in the contact between active spermatozoon and 

 inactive ovum; and that it is the electrical change so set up, and 

 almost instantaneously propagated, which precludes the entry of 

 another spermatozoon J. Whatever the force may be, it is one 

 which acts normally to the surface of the ovum, and after entry the 



* Cf. N. Rashevsky, Mathematical Biophysics, Chicago, 1938; and many earlier 

 papers. Eg. Physico-matheraatical aspects of cellular multiplication and de- 

 velopment, Cold Spring Harbor Symposia, ii, 1934; The mechanism of division of 

 small liquid systems which are the seat of physico-chemical reactions, Physics, in, 

 pp. 374-379, 1934; papers in Protoplasma, xiv-xx, 1931-33, etc. 



t With the classical account by H. Fol, C.R. lxxxiii, p. 667, 1876; Mem. Soc. 

 Phys. Geneve, xxvi, p. 89, 1879, cf. Robert Chambers, The mechanism of the entrance 

 of sperm into the star-fish egg, Journ. Gen, Physiol, v, pp. 821-829, 1923. Here 

 a delicate filament is said to run out from the fertilisation -cone and drag the 

 spermatozoon in; but this is disputed and denied by E. Just, Biol. Bull. LVii, 

 pp. 311-325, 1929. 



I But, under artificial conditions, "polyspermy" may take place, eg. under 

 the action of dilute poisons, or of an abnormally high temperature, these being 

 doubtless also conditions under which the surface-tension is diminished. 



