THE MOSAIC STAGE OF DIFFERENTIATION 217 



a clear crescent, and a large amount of yolk. The first cleavage takes 

 place in a plane passing through the centre of the yellow and clear 

 crescents, and in each blastomere the clear cytoplasm displaces the 

 yolk from the animal hemisphere so that the latter now occupies 

 the vegetative hemisphere. Immediately opposite the yellow 

 crescent, and therefore marking the antero-dorsal side, a third 

 region termed the grey crescent makes its appearance, containing 

 slaty-blue coloured yolk. Eventually, the yellow crescent shows a 

 darker and a lighter coloured region, and there are then at least six 

 different organ-forming substances, which become sorted out 

 between the various blastomeres during cleavage. The determina- 

 tions which these substances represent are the following :^ 



Clear cytoplasm . . . Ectoderm 



Dark yellow cytoplasm Muscle 



Light yellow cytoplasm Mesenchyme 



Yolk region Endoderm 



Slaty-blue Notochord and neural plate 



This distribution can be seen in normal cleavage; the causal 

 connexion between the substances and the organs to whose rudi- 

 ments they are distributed has been proved by the experiments 

 involving killing and disarranging of blastomeres, referred to in 

 Chap, v (pp. 97, 123). 



As mentioned in Chap, v (p. 124), the visible inclusions in the 

 various regions of cytoplasm, such as mitochondria, yolk, etc., are 

 not themselves organ-forming substances, but merely cytological 

 indices of the organisation of the egg.^ In many other cases par- 

 ticular regions of the cytoplasm may be distinguished by their 

 pigmentation, but it can in most cases be shown that the visible or 

 coloured elements do not represent any qualitative determination. 

 The egg of the sea-urchin Arbacia contains fat, yolk, pink granules, 

 and clear cytoplasm, and as these materials differ in their specific 

 gravities, they can be disarranged by the centrifuge. When the eggs 

 of Arbacia are centrifugalised for 5 minutes at 10,000 revolutions 

 per minute, the contents are stratified into four zones, quite regard- 

 less of the original egg-axis, which, it is found, may come to occupy 

 any position in the centrifuge tube. Centripetally (with reference 



1 Conklin, 1905, 1906, 1924, I93i- ^ Conklin, 193 1. 



