NINTH LECTURE. 



THE RATE OF CELL-DIVISION AND THE 

 FUNCTION OF THE CENTROSOME. 



A. D. MEAD. 

 BROWN UNIVERSITY, PROVIDENCE, R. I. 



THERE are few phenomena that bring us so close to the 

 fundamental problems of organic development as do those 

 which relate to the origin of the egg and the spermatozoon, to 

 the union of these cells in fertilization, and to the early divi- 

 sions of the fertilized egg-cell. The egg and the spermatozoon 

 represent the manifold qualities of two separate individuals, 

 and by their union a new individuality is established. In the 

 form and arrangement of the cells into which this oosperm 

 divides, we can recognize the rudiments of the adult body often 

 before the cells become too numerous to be counted; indeed, 

 in many animals the early cleavage-cells constitute a free 

 swimming larva of specific form and possessed of definite 

 functional cellular organs, before the constituent cells are 

 seventy in number, and within four or five hours after the egg 

 is fertilized. 



When the organism is composed of so few cells, it is obvious 

 that the specific form of the body, the size and relations of 

 its organs, is directly dependent upon the size of every com- 

 ponent cell in comparison with the others, upon the position 

 which the several cells occupy in the whole aggregation, and 

 upon the number of cells which perform the same function and 

 constitute a particular organ. The cells which compose the 

 body at a later period of development are but the lineal 

 descendants of those which compose the early larva, and the 



