350 



A. RICHARDS. 



occur. Stockard has made good use of the conception of differ- 

 ential rate of division in his recent paper which elaborates a 

 hypothesis to account for the production of monsters upon this 

 basis. Numerous investigators have reasoned that mitoses occur 

 periodically, and that the duration of the periods may be influenced 

 by various external factors. But these investigations leave un- 

 answered many questions and raise many problems as to the rate 

 of division that merit closer study. What is the normal rate of 

 division for various tissues? What is the duration of each stage 

 of mitosis? Do mitotic cycles go in waves? To what extent do 

 all the cells of a given tissue divide simultaneously? If divisions 

 are not synchronous, what changes in the mitotic ratio occur? 

 What factors influence the rate of cell division ? What are the 

 limits of its modifiability ? 



One of the earliest attempts to study the rate of cell division 

 resulted in the formation of the well-known " Balfour's law of 

 cleavage." This law states in effect that the rate of cleavage is 

 inversely proportional to the amount of deutoplasm that is con- 

 tained within the dividing cell. Wilson's criticism of this law 

 ("The Cell," p. 366) is all that is necessary to show that the 

 formulation of it made no real addition to our knowledge of the 

 subject. He said : " The entire inadequacy of this view has been 

 demonstrated by a long series of precise studies on cell lineage, 

 which show that while the large deutoplasm bearing cells often do 

 divide more slowly than the smaller protoplasmic ones, the reverse 

 is often the case, while remarkable differences in the rhythm of 

 division are often observed in cells which do not appreciably differ 

 in metaplasmic content. All the evidence indicates that rhythm of 

 divisions is at bottom determined by factors of a very complex 

 character which can not be disentangled from those which control 

 growth in general. Lillie ('95, '99) points out the very interesting 

 fact, determined through an analysis of the cell lineage of molluscs 

 and annelids, that the rate of cleavage shows a direct relation to 

 the period at which the products become functional. Thus in 

 Unio the more rapid cleavage of a certain large cell ("D. 2"), 

 formed at the fourth cleavage, is obviously correlated with the 

 early formation of the shell gland to which it gives rise, while the 

 relatively slow rate of division of the first ectomere quartet is 



