THE MITOTIC CYCLE 



comparatively rigid. Mirsky^^^ finds that the proportion of protein 

 soluble in molar potassium chloride within the egg is decreased on 

 fertilization from about 83 per cent to about 70 per cent; this is cor- 

 related with a decrease in the proportion of bound calcium by about 

 15 per cent (Mazia^^^). This observation forms part of the evidence 

 for a general theory of protoplasmic stimulation which has been 

 advanced by Heilbrunn and his colleagues (Heilbrunn^^i), some 

 comments on which have recently been made by Danielli.^^^^ 

 According to this theory, the reaction of the living cell to a stimulus is 

 to release calcium from the cortex into the interior, where the height- 

 ened concentration of this ion provokes an increased viscosity, which 

 Heilbrunn calls 'protoplasmic clotting'. The effect of calcium in 

 decreasing surface permeability is well known. This general theory has 

 been developed by Heilbrunn in relation to the effects of various stimuli 

 on amoeboid movement, on protoplasmic streaming in plant cells, and 



^^ 



I \ \ 1^ 



20 30 



Time 



W 



Figure 52. Curves of viscosity against 

 time during first cleavage period of egg 

 of Arbacia punctulata obtained by centri- 

 fugal sedimentation. A in pigment zone 

 of cortex at 408 atmospheres; B, Heil- 

 brunn's curve for whole egg. In A, the 

 arbitrary units are verv much larger than 

 in B. {From original of fig 4 by Dr. D. E. S. 

 Brown, J. Cel. camp. Physiol, 5 (1934) 342. 



on muscle and nerve fibres. The evidence is most detailed for the 

 stimulus of fertilization, and it is tempting to inquire whether similar 

 events occur during division of the egg cell. There are close analogies 

 between the events which follow fertilization and those which occur 

 during the later stages of mitosis, in respect to the astral cycles, the 

 changes in viscosity and permeability, and also in the birefringence and 

 light scattering properties of the egg membrane (Swann and Mitchi- 

 50n322)_ Xhe refractive index of the whole egg, according to Vles^^^ 

 falls after fertihzation, and rises to a maximum just before cleavage 

 (Figure 53). Oddo and Esposito^^^^ have shown that an uptake of 

 potassium follows fertilization in both Arbacia and Paracentrotus. This 

 event is followed by a loss of this ion, which again is succeeded by an 

 uptake reaching a maximum immediately before cleavage. 



Heilbrunn and Wilson^^* have proposed the generalization that 

 protoplasmic clotting is the primary cytoplasmic change in mitosis 

 and have observed that heparin prevents both mitotic gelation and cell 

 division in the eggs of Chaetopterus. To this subject we shall return in a 

 later chapter (p 194). It would be of interest to know whether any 

 change in the proportion of free and bound calcium occurs at cleavage, 



140 



