CLEAVAGE AND DIFFERENTIATION 123 



differentially distributed by a specialised cleavage, or regulation 

 may be prevented by a high degree of viscosity in the egg, at any 

 given stage. Bero'e affords an example of the latter method, but the 

 best illustration is provided by the Ascidians. 



It will be remembered that the Ascidian Qgg is in a liuid state 

 before fertilisation, but that after this event its viscosity is enor- 

 mously increased. Further, we have already mentioned the localisa- 

 tion of different organ-forming substances which takes place at 

 fertilisation. Elaborate experiments on the effects of killing blasto- 

 meres and of centrifugalisation have shown that the fertilised egg 

 is already a mosaic of chemo-differentiated regions. 1/2, 1/4, and 

 3/4 blastomeres all develop into those parts of a larva to which they 

 would have given rise in normal development. The partial embryos 

 round themselves off, and this process in some forms (e.g. Phallusia) 

 goes much further than in others (e.g. Styela, Ciona), so that the 

 products of single blastomeres may appear superficially to be whole 

 larvae, but sections invariably show that they are only parts, halves 

 or quarters (e.g. with mesoderm only on one side; see p. 97 and 

 figs. 45, 59). The egg of Amphioxus behaves in an extremely similar 

 manner.^ In the experiments on Amphioxus, the blastomeres of the 

 2- and 4-cell stages were frequently disarranged without being totally 

 separated. In such cases, they always preserved their inherent 

 polarity, though there was complete fusion between their products. 

 The result was the formation of double monsters in various orienta- 

 tions (fig. 34). 



In Styela, the fertilised egg contains yolk in the animal hemi- 

 sphere, cytoplasm with yellow mitochondrial granules at the 

 vegetative pole, and clear cytoplasm in between. In the centrifuge 

 tube the eggs tend to orientate themselves in such a way that the 

 animal pole with the relatively heavy yolk is centrifugal, and the 

 stratification of the egg is then increased by the centrifugalisation. 

 But if the eggs are slightly compressed, either by mutual pressure, 

 or by being placed in fine tubes so that they cannot rotate, centri- 

 fugalisation can restratify the egg-contents in such a way that, for 

 instance, all the yellow granular cytoplasm is confined to one of 

 the blastomeres of the 2-cell stage. The resulting embryo then 

 possesses muscle-fibres only on one side of the body.^ The mosaic 



^ Conklin, 1933. - Conklin, 1924, 193 1. 



