CLEAVAGE AND DIFFERENTIATION 107 



Stages, the green substance is largely localised at one end of the 

 blastomeres. During this period the cytoplasm is highly viscous : 

 it then becomes more fluid, and the green substance is redistributed 

 uniformly round each blastomere. After the 8-cell stage, however, 

 it remains localised near one pole, and is progressively separated 

 off by a series of unequal cell-divisions into the micromeres. 



If the egg is cut at stages up to the 8-cell stage, the result will 

 depend on two factors : first, whether the distribution of the green 



D 



Fig. 50 



Mosaic development of Ctenophores. A, B, Fragments of i6-cell stage, divided 

 unequally, so that A has five macromeres and five micromeres; B, three macro- 

 meres and three micromeres. C, D, Partial larvae developed from A and B, 

 respectively; C has five costae, D has three. (After Fischel, from Schleip, Z)£'/^r- 

 mination der Primitiventwicklung, 1929, fig. 19, p. 52.) 



substance at the moment is uniform over the surface of the blasto- 

 meres, or if it is asymmetrically localised ; and secondly, whether 

 the egg is in a more fluid state when redistribution of the green 

 substance is easy, or in a very viscous state when redistribution 

 may be impossible before the next cleavage. These facts account 

 for the certain amount of regulation which has been obtained in 

 some experiments on Ctenophore eggs. Immediately after being 

 laid, the tgg of Bero'e is in a highly viscous state, but with the ap- 

 proach of the first cleavage division it becomes more fluid. If in 



