SPIRALLY CLEAVING EGGS lOI 



Janus Green, Neutral Red, etc.) often show different colours in the differ- 

 ent regions of mosaic eggs ; usually these differences are related to the 

 main animal-vegetative axis. In typical regulation eggs, such as echino- 

 derms, the differences are absent or at least much less w^ell developed, 

 but the same is also true in some mosaic eggs, where they would be 

 expected. It is not easy to give an exact interpretation of the phenomena, 

 since it is notorious that when indicator dyes are in the presence of pro- 

 tein, their behaviour is atypical, and one cannot simply deduce the pH or 

 rH from the colours which they take up. Nevertheless, the existence of 

 differences in colour certainly demonstrates the presence of some differ- 

 ences or other in the cytoplasm, even if it is unsafe to conclude much as to 

 their nature. Histochemical tests for various enzymes (e.g. indophenol- 

 oxidase), or fixed -SH groups, ascorbic acid, etc., have also revealed 

 certain cases in which these substances are strictly localised within mosaic 

 eggs. The subject has recently been reviewed by Needham (1942, p. 131 

 seq.) and Brachet (1944, p. 271, seq.). It will be seen from their discussions 

 that the biochemical interpretation of the findings is not clear in this 

 instance also. In particular, the suggestion of Ries (1942), who has been 

 one of the most active workers in this field, that the most important 

 differences between the regions of mosaic eggs are related to the intensities 

 of respiration, is almost certainly based on too optimistic a neglect of the 

 possible sources of uncertainty. Nevertheless it is important that a begin- 

 ning at least has been made with the biochemical recognition of cytoplas- 

 mic localisations. 



It has also been possible to obtain some information about the physical 

 properties of the substances involved in the mosaic regions. One may 

 ask whether they are small-molecular substances, such as amino-acids 

 or vitamins; or on the other hand are they larger entities which should 

 be regarded as cell constituents rather than chemical molecules ? The evi- 

 dence which has been available for some time, that they can be shifted 

 about witliin the egg-cell by centrifugation, provides grounds for pre- 

 ferring the second alternative. Recently Lehmann and Wahli (1954) have 

 made a careful study with the electron microscope of the structure of the 

 cytoplasm in the various blastomeres of Tubifex. They find that at a fairly 

 early stage there are characteristic quantitative differences between the 

 cells. Thus by the time the four sets of micromeres have appeared, the cell 

 2d (which will produce the embryonic ectoderm) has more of the baso- 

 philic fibrillar cytoplasm, v^th many small globular particles and pecuHar 

 spindle-shaped bodies, whereas id (which gives the embryonic meso- 

 derm) has sparse fibrillar cytoplasm, few of the globular or spindle-shaped 

 bodies but many of a larger type of particle which can be designated 



