CLEAVAGE 



77 



uncleaved egg. It seems that it must have arisen de novo. Schechtman 

 (1937) suggested that it is formed by the growth of the cortex into the 

 depth of the furrow. Mitchison and Swann argue that such growth takes 

 place, not merely from the very edge of the original cortex, but by intus- 

 susception of new material throughout the whole area of the infolding 

 region; if this were so, the process becomes in effect a type of cortical 

 expansion. 



A similar increase in surface area occurs in the furrow region of the 

 echinoderm, but in this form it seems to take place after the cleavage 

 plane has cut through the egg (Fig. 4.10). According to Dan (1954) 



\ .' <■• >CTV ra^^w xjf^iz • •« 



Figure 4.10 



A. Section through part of a newt's egg at an early stage in the first cleavage. 

 The future surface which will separate the two blastomeres is already indi- 

 cated in the internal cytoplasm although there is not yet any furrowing of 

 the external surface in this region. (After Selman and Waddington 1955.) 

 J3. Three stages in the cleavage of an echinoderm egg. Small kaolin particles 

 (indicated by short lines) have been placed on the surface. Note that they at 

 first move into the furrow (2), but that later new cortex is produced in the 

 depth of the furrow (dotted), so that the particles move out again. (From 



Dan 1948.) 



it also seems to be due to intussusceptive growth, accompanied by 

 stretching. From this evidence, Mitchison and Swann conclude that the 

 cortical expansion involved in their theory always includes an element 

 of intussusceptive growth. But even if this is so, the amount of growth as 

 compared with mere stretching must be so vastly greater in the furrow 

 region than elsewhere as to amount to a qualitatively different type of 



