66 PRINCIPLES OF EMBRYOLOGY 



cell membrane cutting through a spindle before the separation of the 

 chromosomes has occurred. The dependence of cell division on nuclear 

 phenomena is, however, not absolute, since it can occur in parts of cells 

 v^hich contain no nucleus (p. 64). But in these anucleate cells, centrosomes, 

 if missing, arise de novo, and it seems that they control the occurrence of 

 the cleavage ; thus even in this case the cytoplasmic division is not v^holly 

 independent of the behaviour of the achromatic apparatus. 



The nature of the connection betw^een the nuclear division and the 

 cleavage of the cell body is not well understood. Swarm (195 1, 1952) 

 has showed by studies with the polarising microscope that the orderly 

 arrangement of the material which forms the asters at each pole of the 

 spindle decreases as the chromosomes come into their neighbourhood at 

 anaphase. He suggests that this is brought about by a substance released 

 from the chromosomes, and that this substance diffuses away from the 

 two daughter-nuclei until it reaches the cell cortex, where it initiates the 

 processes leading to cell cleavage. It would seem that some diffusing 

 agent of this kind must almost certainly be involved, but it is not clear 

 that it arises from the chromosomes. 



Darlington (1937) has drav^n attention to the type of cell cleavage which 

 occurs in cases where the movement of some of the chromosomes on the 

 spindle has been abnormal. He claims that if one or two chromosomes 

 have lagged behind the others and become included in a small separate 

 nucleus of their own, a plane of cell division often forms around this 

 nucleus, but only in those cases in which the chromosomes are provided 

 with centromeres : the cleavage planes pay no attention to acentric frag- 

 ments. Again, it is well known that if two homologous chromosomes 

 become joined together (e.g. by chiasmata inside an inversion) and are 

 unable to separate properly at anaphase, they form a 'chromosome bridge' 

 comiecting the two telophase nuclei, which move away from one another 

 as far as the connection will allow. The cleavage plane then forms between 

 the two nuclei but is unaffected by the chromosome bridge and cuts 

 through it as though the connecting chromosome material were not 

 present. From tliis type of evidence Darlington concludes that it is the 

 centrosomes and centromeres which affect cell cleavage rather than the 

 chromosomes themselves. 



The first factor, then, which plays a part in the determination of the 

 cleavage pattern of an egg is the orientation and position of the cleavage 

 spindles which initiate the changes in the cell body. In spirally cleaving 

 eggs, for instance, from the 4-cell stage onwards the cleavage spindles 

 are obliquely inclined, first to one side and then to the other of the vertical, 

 and this gives rise to the characteristic pattern of the group of cells. The 



