380 DEVELOPMENT 



Attempts have been made to determine what part the sperma- 

 tozoon plays in the process of fertilisation. Brailsford Robertson 

 has extracted a substance oocytin from the testicles of the sea- 

 urchin which produces membrane formation. The question then 

 arises " Is this substance a catalyst speeding up some slow 

 change or does it counteract some obstacle to development ? " 

 To the first part of the question a negative answer can be given 

 The velocity of the process of development is not catalytic. It 

 does not follow Schiitz's law and vary in velocity with the square 

 root of the concentration. If two spermatozoa are caused to 

 enter an ovum, the rate of segmentation should increase by 1-4 

 (i.e. >/2) times, if the process were catalytic. The rate, as a 

 matter of fact, is unaltered by the introduction of additional 

 spermatozoa. Therefore, the spermatozoon does not contain a 

 catalyst for developmental processes. 



Loeb is inclined to believe that the spermatozoon removes 

 from the egg a substance or condition which inhibits or prevents 

 the process of development. 



On the other hand, it is conceivable that the entry of the 

 spermatozoon increases the free energy of the now fertilised ovum. 

 The potential energy of the system cannot be utilised without 

 the employment of a small quantity of free energy. This quantity 

 of free energy may be extraordinarily small as long as it is sufficient 

 to start the series of reactions which once started are autocatalytic. 



One result of the entrance of an effective spermatozoon into 

 an ovum is an acceleration of the processes of oxidation, i.e. 

 metabolism begins, and the various phases of development can be 

 followed by the same calorimetric methods (direct and indirect) 

 adopted in the study of the energy exchanges of the mature 

 organism (see over, p. 386). 



Differentiation. 



Cell division is the most general of the specific functions of 

 living protoplasm and it is the basis underlying the differentiation 

 of the comparatively simple structure of the egg into a more 

 complex organism. The division of a cell is a necessary conse- 

 quence of its increase in volume. The metabolic activity of the 

 cell is a function of its effective surface, i.e. its surface must be 

 of such a size compared with its volume that an adequate supply 

 of oxygen can reach the centre of the cell and that all the by- 

 products of cellular activity can be eliminated with sufficient 

 rapidity. A freely suspended unicellular animal is spherical. 



