CLEAVAGE 75 



second. The second substance, on the other hand, leaves these processes 

 relatively unaffected or even exaggerates them, but the cells tend to fail 

 actually to divide into two, so that one finds, for instance, an elongated 

 but undivided CD cell (Fig. 4.8). Huber therefore concludes that cell 

 division involves two different cortical movements, not only the expan- 

 sion seen in the formation of protuberances, but also a contraction in the 

 furrow region. 



A consideration of more yolky types of egg would also suggest that 

 a mere expansion of the already existing cell cortex cannot be the only 

 factor causing the cleavage. In the extremely yolky eggs of a sturgeon, 



FiGXJRE 4.8 



The effect of quinones on cleavage : a, section through a 2-cell stage of the 

 egg of Tuhifex; the nuclei are aheady in telophase of the second division and 

 the CD cell (below) is markedly elongated preparatory to dividing in two ; 

 b, a similar stage from an egg treated with 1,4-Naphthoquinone, note the 

 slight elongation of the CD cell; c, after treatment with 9,10-Phenanthrene- 

 quinone the elongation of the CD cell is exaggerated. (After Huber 1947.) 



for instance, the cleavage furrows eventually extend a long way from the 

 position of the spindle, and it is difficult to see how the influence of a 

 cortical expansion starting in the neighbourhood of the spindle poles 

 could reach so far (cf Fig. 4.1). Moreover in such forms the second cleav- 

 age furrow starts to form in the animal region while the first furrow is still 

 working its way down towards the vegetative part of the egg. It seems 

 unhkely that any system of expansions working through the cortex as 

 a whole could control the furrows in such cases. It would be much easier to 

 attribute the formation of the cleavage planes to factors located in the 

 immediate neighbourhood of the furrows themselves. 



Studies on the moderately yolky eggs of the Amphibia suggest, indeed, 

 that in them also cortical expansion is not such an important factor in 

 cleavage as Mitchison and Swann suppose it to be in the smaller eggs of 

 the echinoderms. Selman and Waddington (1955) have shown that there 



