CLEAVAGE 69 



tion the egg becomes much more elongated, with a given degree of 

 centrifugation, than it does when the force acts along the egg axis; this 

 presumably indicates that the egg cortex is more easily deformed in the 

 equatorial than in the animal-vegetative plane. When second polar-body 

 formation begins, protuberances appear in the equatorial region of the 

 egg in the neighbourhood of the spindle. They thus form a bilaterally 

 symmetrical pattern very similar to that characteristic of the normal first 

 cleavage, in which again it is an equatorial region of cortex which is 

 closest to the initiating spindle. It is clear, then, that the protuberances are 





vi^'w 



"N J rV / .V P- -. J%..^>d^^ .. -;li 



J> f^ 



\K 



W^i 



FiGUKE 4.5 



Maturation and first division of the egg of Tubifex, seen from the animal 

 pole ; a, soon after laying ; b, formation of protuberances during the extru- 

 sion of the first polar body ; c, between first and second polar body divisions ; 

 d, extrusion of the second polar body ; e, before first cleavage of fertiUsed 

 egg (the animal pole plasm shaded) ; /, early stage of cleavage, pole plasm 

 elongated;^, h, stages of cleavage showing protuberances; i, two cell stage 

 (the AB blastomere above, CD below). (After Woker 1944.) 



produced by an interaction between the cleavage spindle and the cortex 

 in its neighbourhood, which has a structure which differs in the different 

 parts of the egg. Lehmann claims that this structure is to some extent 

 visible in the living egg, which shows a pattern of nine to fifteen sub- 

 cortical meridianal thickened strands of the heavy fibrillar type of cyto- 

 plasm which he names 'plastin'. This material is, however, at least to some 

 extent, shifted by the centrifugation, and the structure which persists in 

 the cortex of the centrifugal eggs is perhaps not due to the plastin, but to 

 some associated structure in the cortex itself. 



