194 ON THE INTERNAL FORM AND [ch. 



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 lie 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 hne 

 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 httle streak of brown 

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

 reaching the centre has been repeatedly seen by many observers. 

 The problem is nothing else than a particular case of the 

 famous problem of three bodies, which has so occupied the 

 astronomers ; and it is obvious that the foregoing brief description 

 is very far from including all possible cases. Many of these are 

 particularly described in the works of Fol, Roux, Whitman and 

 others*. 



The intracellular phenomena of which we have now spoken 

 have assumed immense importance in biological hterature and 

 discussion during the last forty years ; but it is open to us to doubt 

 whether they will be found in the end to possess more than a 

 remote and secondary biological significance. Most, if not all of 

 them, would seem to follow Immediately and inevitably from very 

 simple assumptions as to the physical constitution of the cell, and 

 from an extremely simple distribution of polarised forces within 

 it. We have already seen that how a thing grows, and what it 

 grows into, is a dynamic and not a merely material problem; so 

 far as the material substance is concerned, it is so only by reason 



* Fol, H., Recherches sur la fecondation, 1879. Roux, W., Beitrage zur 

 Entwickelungsmechanik des Embryo, Arch. f. Mikr. Anat. xix, 1887. Whitman, 

 C. O., Ookinesis, Journ. of Morph. i, 1887. 



