SPIRALLY CLEAVING EGGS IO3 



Lehmann (1948(2,), who has described very similar phenomena in the 

 ohgochaete Tuhifex, beheve that this control is exerted by the cortex. 

 They suggest that the polar regions of the cortex are not disturbed by 

 centrifugation, and that they exert specific attractions on the materials 

 vv^hich should form the appropriate pole plasms, so that these cytoplasmic 

 localisations can become reconstituted. This suggestion w^ould tend to 

 suggest similarities between the mosaic spiral-cleaving eggs and the 

 typical regulation eggs of the ecliinoderms, in which we have seen that 

 the cortex is probably the seat of the epigenetic gradients which play the 

 main part in early development in those forms. It might be, indeed, that 

 in the spiral eggs also the difference between the animal and vegetative 

 cortical regions is graded, as it is in the echinoderms, but that in the 

 former this cortical gradient is very rapidly converted into a qualitative 

 distinction by the attraction of definitely different materials to form the 

 two pole plasms. 



Although an attraction between cortical regions and particular types 

 of cytoplasm may be of major importance in controlling localisation in 

 mosaic eggs, it seems doubtful whether it can be the whole story. The 

 restoration of normality in a centrifuged egg is not merely a matter of 

 attracting one specific substance to each end of the original axis, but of 

 redistributing the whole set of substances which have been deranged. 

 Costello (1948), another recent worker in this field, argues that there must 

 be a more generally pervasive system of relations which orders the egg 

 cytoplasm throughout its mass. He refers to this as 'ooplasmic segrega- 

 tion' and has advanced an ingenious hypothesis as to its nature, based on 

 the idea of diffusion gradients. It seems not impossible that his ideas and 

 those of Raven and Lehmann will eventually come together; perhaps the 

 polar cortical regions establish the initial difference between the two ends 

 of the main axis, and this is transmitted through the mass of the egg by 

 some diffusion mechanism similar to that which Costello has indicated. 



We have already seen that the various regions which become localised 

 in so-called mosaic eggs are not quite so rigidly determined in their 

 developmental fate as the word 'mosaic' originally impHed. It is also 

 important to realise that in these eggs other mechanisms are at work, 

 which are quite different from the localisation of distinct types of cyto- 

 plasm. Raven (1952) has made a particular study of them in Limnea. He 

 found that treatment with lithium, at stages earlier than the twenty-four 

 cell, caused a reduction in the head. The medio-dorsal part was most 

 strongly affected, and the effects show a series of grades leading to com- 

 plete disappearance of this region and fusion of the eyes. This is a typical 

 example of the type of behaviour spoken of as a 'field' process ; that is 



