268 



CELL-DIVISION AND DEVELOPMENT 



of the divisions is endlessly varied by special conditions. These 

 modifications are all referable to the three following causes : 



1. Disturbances in the rhythm of division. 



2. Displacement of the cells. 



3. Unequal division of the cells. 



The first of these requires little comment. Nothing is more com- 

 mon than a departure from the mathematical regularity of division. 

 The variations are sometimes quite irregular, sometimes follow a 

 definite law, as, for instance, in the annelid Nereis (Fig. 122), where 

 the typical succession in the number of cells is with great constancy 



Fig. 120. Cleavage of the ovum in the holothurian Synapta (slightly schematized). [After 

 SELENKA.] 



A-E, Successive cleavages to the 32-cell stage. F. Blastula of 128 cells. 



2, 4, 8, 16, 20, 23, 29, 32, 37, 38, 41, 42, after which the order is more 

 or less variable. The meaning of such variations in particular cases 

 is not very clear. They are certainly due in part to variations in the 

 amount of deutoplasm ; for, as Balfour long since pointed out ('75), 

 the rapidity of division in any part of the ovum is in general inversely 

 proportional to the amount of deutoplasm it contains. Exceptions 

 to this law are, however, known. 



The second series of modifications, due to displacements of the 

 cells, are probably due to mutual pressure, however caused, 1 which 



1 The pressure is probably due primarily to an attraction between the cells (cytotrophm 

 of Roux), but may be increased by the presence of membranes, by turgor, or by special 

 processes of growth. 



