78 PRINCIPLES OF EMBRYOLOGY 



phenomenon. Moreover, Selman and Waddington find that sections of 

 an amphibian egg in the process of division show that an indication of 

 the future cleavage plane is developed in the cytoplasm over a much 

 wider area than corresponds to the externally visible furrow. It seems very 

 unlikely that the material making up this precursor of the division plane 

 is directly derived from the original cortex. It looks rather as if it differen- 

 tiates in situ (although it must be the cortex which initiates the process of 

 differentiation). It gradually increases in extent, and also in thickness, and 

 eventually spHts into two separate films which form the outer membranes 

 of the two blastomeres in the region where they are in contact. If the 

 vitelline membrane is removed, so that there is nothing to hold the two 

 cells together, they tend to fall apart as they settle down on the bottom 

 of the dish, and the white newly formed cell membrane is then extensively 

 exposed in the depths of the furrow as one looks at the egg from the 

 animal pole; it may be visible within the furrow even in eggs cleaving 

 inside the membrane. 



The evidence available at the present time would seem, therefore, to 

 suggest that several factors are operative in the process of cell cleavage, 

 their relative importance differing in various groups of animals. These 

 factors are: firstly, localised expansions of the cortex; secondly, an increase 

 in stiffness of the cortex; thirdly, in the Ampliibia at least, an increase in 

 the tangential force acting in the cortex; fourthly, a contraction localised 

 in the neighbourhood of the furrow and directed along its length; fifthly, 

 a formation of new cell membrane from the sub-cortical cytoplasm. It is 

 possible that Swann and Mitchison are correct in ascribing the major 

 importance to the first two of these in the echinoderm egg but it seems 

 likely that in other eggs, particularly those with large quantities of yolk, 

 the other three factors have to be taken into account. 



SUGGESTED READING 

 Fankhauser 1948, Mitchison 1952, Swann 1952, Lehmann 1946, 



