336 ON THE INTERNAL FORM [ch. 



spermatozoon points straight towards the centre of the egg. From 

 the fact that other spermatozoa, subsequent to the first, fail to 

 effect an entry, we may safely conclude that an immediate con- 

 sequence of the entry of the spermatozoon is an increase in the 

 surface-tension of the egg: this being but one of the complex 

 reactions exhibited by the surface, or cortex of the cell*. Some- 

 where or other, within the egg, near or far away, lies its own nuclear 

 body, the so-called female pronucleus, and we find that after a 

 while this has fused with the "male pronucleus" or head of the 

 spermatozoon, and that, the body resulting from their fusion has 

 come to occupy the centre of the egg. This must be due (as Whitman 

 pointed out many years ago) to a force of attraction acting between 

 the two bodies, and another force acting upon one or other or both 

 in the direction of the centre of the cell. Did we know the magnitude 

 of these several forces, it would be an easy task to calculate the 

 precise path which the two pronuclei would follow, leading to con- 

 jugation and to the central position. As we do not know the 

 magnitude, but only the direction, of these forces, we can only make 

 a general statement: (1) the paths of both moving bodies will he 

 wholly within a plane triangle drawn between the two bodies and 

 the centre of the cell; (2) unless the two bodies happen to he, to 

 begin with, precisely on a diameter of the cell, their paths until they 

 meet one another will be curved paths, the convexity of the curve 

 being towards the straight line joining the two bodies; (3) the two 

 bodies will meet a httle before they reach the centre; and, having 

 met and fused, will travel on to reach the centre in a straight hne. 

 The actual study and observation of the path followed is not very 

 easy, owing to the fact that what we usually see is not the path 

 itself, but only a projection of the path upon the plane of the 

 microscope ; but the curved path is particularly well seen in the frog's 

 egg, where the path of the spermatozoon is marked by a little streak 

 of brown pigment, and the fact of the meeting of the pronuclei before 

 reaching the centre has been repeatedly seen by many observers f. 



* See Mrs Andrews' beautiful observations on "Some spinning activities of 

 protoplasm in starfish and echinoid eggs," J own. Morphol. xii, pp. 307-389, 1897. 



t W. Pfeffer, Locomotorische Richtungsbewegungen durch chemische Reize, 

 Unters. a. d. Botan. Inst. Tubingen, i, 1884; Physiology of Plants, m, p. 345, Oxford, 

 1906; W. J. Dakin and M. G. C, Fordham, Journ. Exp. Biol. t. pp. 183-200, 1924. 

 Cf. J. Loeb, Dynamics of Living Matter, 1906, p. 153. 



