CELL-DIVISION AND DEVELOPMENT. 1 43 



nounced; they refer to the final form of the embryo, and are 

 instances of Sachs' law that growth determines division and 

 not division growth. Each stage of the development appears 

 to stand in relation not only to what has preceded it, but to 

 what is to succeed it, and is a link in a chain one end of which 

 is lost in the obscurity of the past while the other stretches 

 forward into the future. We must, I believe, recognize the 

 fact so forcibly discussed by Dr. Whitman in his lecture on 

 the Inadequacy of the Cell-Theory of Development and so 

 clearly shown by centrolecithal ova, that in embryological 

 development the differentiation which occurs is a differentia- 

 tion of the entire organism and not of the constituent parts or 

 cells of which it is composed; physiologically, if not morpho- 

 logically, every organism is a syncytium, and future theories of 

 heredity must take this into consideration. 



From what has been said so far it will, I think, be evident 

 that while the direction of the cleavage spindles and the 

 arrangement of the boundary planes between adjacent cells 

 may in some cases be explained by the action of simple me- 

 chanical laws, many of the peculiarities seen in animal ova 

 cannot be thus accounted for. I do not mean to assert that 

 the causative force which produces these peculiarities is at all 

 different from the forces with which we are already familiar ; I 

 do not mean to say that there is a special vis vitae differing in 

 its nature from the physical and chemical forces which are 

 already known to us ; but the actions and interactions of these 

 forces are far too complicated for us to obtain even a faint 

 conception of them, even as the chemical composition of proto- 

 plasm itself is too complex for us to understand its exact 

 nature and its synthesis. What conception can we form of 

 the forces which cause the aggregation of the peripheral proto- 

 plasm of Porcellio for instance.!* It may be spoken of as a 

 precocious segregation of the cytoplasm of the blastoderm, but 

 can we picture to ourselves the dynamic interactions which 

 bring about this segregation .? We have a resultant here which 

 we cannot yet analyse into the constituent forces. 



And with regard to the later processes of development a 

 mechanical explanation is even more difficult. For instance, 



