APPENDIX A 309 



Roux describes it as being due to the immigration of super- 

 ficial pigment. Now we have strong reason for believing -that 

 both the entrance-funnel produced when the spermatozoon first 

 touches the egg and the sperm -sphere are local aggregations of 

 watery substance. The accumulation of what appears to be a 

 more watery substance about the middle piece which has been 

 observed in the Axolotl, appears also to occur in the Frog : at least 

 the same formation of large clear vacuoles in the sperm-sphere may 

 be seen in the latter as in the former. Should this be actually so, 

 we may suppose that the streaming movement centred in the 

 entrance-funnel and sperm-sphere is responsible for drawing away 

 the pigment from a certain region of the surface ; hence the grey 

 crescent. The sperm-sphere is on the inner side of the sperm 

 'nucleus : hence the grey crescent would appear on that side of 

 the egg which is opposite to the entrance of the spermatozoon, 

 should no disturbance of the streaming movement have taken 

 place, and, since the sperm-path is radial, would be symmetrically 

 disposed with regard to it. In this case, fertilization meridian, 

 sperm-path, grey crescent and plane of symmetry, first furrow, 

 and, later on, sagittal plane, would all coincide. There is, as 

 we have seen, a very fair correlation between the sperm -entrance 

 and the first furrow, and again between the sperm-path and the 

 grey crescent. But should some other streaming movement of 

 the cytoplasm be set up by the gravitation of the heavy yolk 

 particles, or by pressure, or by light, then the relation between 

 the two processes, the division of the centrosome which deter- 

 mines the direction of the first furrow, on the one hand, and 

 on the other, the streaming movement towards the sperm-sphere 

 which determines the position of the grey crescent, would be 

 disturbed, and while the entrance point of the sperm might still 

 continue to determine, though not so completely, the position of 

 the furrow, it might come to be without relation to the symmetry 

 of the egg and of the embryo ; and this is what is actually 

 observed. 



Though it is difficult to assign the exact cause of each and 

 every deviation from the rule, this much is certain, that however 

 they may coincide in 'typical* development (I use Roux's 

 expression), the factors which determine cell-division, and those 

 which determine differentiation, may be influenced by different 

 external causes in widely differing ways, and are therefore pre- 

 sumably distinct. Nor does this artificial separation of the two 

 processes in any wise prejudice the complete normality of the 

 development of the embryo. 



