CLEAVAGE 65 



physiology and we shall confine ourselves here to those aspects which are 

 particularly important in relation to the problems of embryonic develop- 

 ment. We shall therefore pay little attention to the subject of chromo- 

 some movements or the details of the behaviour of the achromatic appara- 

 tus (for which consult textbooks on cytology, such as Darlington 1939, 

 White 1950, 1954, Schrader 1944, Hughes 1952). From the point of view 

 of the embryologist, the important subjects to discuss are the determina- 

 tion of the pattern of cleavage and the mechanism by which the body of 

 the cell becomes divided into two parts. 



Figure 4.3 



Section through the best cleaved portion of a blastula, which was developed 

 from an enucleated egg of the frog Rana pipiens, inseminated by a sperm of 

 R. catesbiana whose nucleus had been inactivated by U.V. irradiation. The 

 cells are outlined by pigment granules and the dark spots resembling nuclei 

 are also accumulations of pigment. There is some chromatin in most cells 

 in region <?, in a few cells in region b, but none in the cells of region c. 

 (From Briggs, Green and King 195 1.) 



The series of events by which a cell is cleaved in two is normally initia- 

 ted by the nuclear division, during which the chromosomes become 

 separated into two daughter groups. That there is a causal relation be- 

 tween the two processes is shown by the general concordance in their 

 timing : nuclear division is usually followed immediately by cell division. 

 The primacy of the former is shown by the fact that when, in abnormal 

 cells or under experimental conditions, the usual sequence is disturbed, 

 it is the cleavage of the cell rather than the division of the nucleus which is 

 most easily changed from its normal course. Thus it is not uncommon for 

 nuclear divisions to occur without any following cleavage of the cell 

 body, but it is very rare for the cell to cleave in a way which is not depen- 

 dent on the events proceeding in the nucleus. We do not, for instance, find 

 a cell cleaving before the nucleus has entered into its division process, or a 



