70 PRINCIPLES OF EMBRYOLOGY 



There is normally also an accumulation of plastin around the nucleus. 

 With inild centrifugation in a polar direction at a stage shortly after fertihs- 

 ation, all the plastin is driven to the centrifugal end while the nucleus still 

 remains near the centripetal end, surrounded only by yolky cytoplasm. 

 In such eggs the nucleus shows no sign of entering into division, which 

 presumably indicates that the nuclear processes are normally initiated by a 

 reaction which involves the surrounding plastin material. Further, no 

 deformations of the cell cortex occur, which again demonstrates that 

 they are initiated by the nuclear division process. These interactions be- 

 tween cortex and nucleus in Tubifex are somewhat reminiscent of those 

 between the nucleus and the cortical granules shortly after fertihsation in 

 the echinoderms, as described by Allen (p. 51). 



It appears, then, that the pattern of cleavage is determined by inter- 

 actions between the spindle and the cortex. We have now to consider the 

 nature of the forces which bring about the deformation of the cell and its 

 division into two. There are a number of theories in the field. As the first 

 group we may take those which suppose that the nuclei and spindle 

 apparatus continue to play in important part throughout the whole 

 course of the division. For instance, Gray (Review: 193 1), from studies on 

 the sea-urcliin egg, suggested that the asters continue to grow until all 

 the available cytoplasm is incorporated into two spheres of radially 

 arranged material at the poles of the spindle, and that the cortex passively 

 accommodates itself to these two masses, which form the two first 

 blastomeres. Dan (Review: 1948), working on the same form, suggests 

 that the astral rays are actually attached to the cortex, and he shows by 

 means of a model how, if this were so, a contraction of the rays would 

 cause the bending in of the cortex and at least the beginning of the process 

 of division. However, these mechanisms certainly do not operate in all 

 types of eggs, if indeed in any. For instance, cleavage in the amphibian 

 egg can continue quite satisfactorily when a large amount of the internal 

 contents has been removed so that the cortex is quite flaccid and the asters 

 imable to produce any internal turgor. Moreover, after a cleavage furrow 

 has begun to form it can extend over a region of cortex which has been 

 isolated from the spindle by the insertion of a strip of cellophane, which 

 must certainly prevent the attachment of any astral rays (Waddington 

 1952^; Mitchison 1953 has similar evidence in echinoderms). (Fig, 4,6.) 

 Finally Swann and Mitchison (1953) fmd that if sea-urchin eggs are treated 

 with colchicine at anaphase, when the chromosomes have separated but 

 the division of the cell body has not yet begun, the asters disappear but the 

 cleavage occurs normally. Thus it seems fairly certain that although the 

 division spindle initiates the process of cleavage it does not play any straight- 



