CLEAVAGE AND DIFFERENTIATION IO5 



whole larva, but would have been forced to continue development 

 as a part. 



It appears, in point of fact, that one of the reasons for mosaic 

 development from egg or blastula fragments is extreme viscosity 

 of the cytoplasm. 



Il)||||lil(l|!l!;!lll!lllillllll 



Fig. 48 

 Diagram to show the influence of the primary axial gradient in fusion- experi- 

 ments with sea-urchin eggs. Left, single egg and resultant pluteus. Centre, two 

 eggs united with their axes parallel produce a single pluteus. Right, two eggs 

 united with their axes at an angle produce double monsters. (From Przibram, 

 Handb. norm, and pathol. Physiol, xiv, 1925, fig. 411, p. 1099.) 



A good example of this is found in Ctenophores. Here the adult 

 has eight swimming plates or costae. But although in these forms 

 the first two cleavage divisions are meridional, larvae developed 

 from 1/2 blastomeres have only four costae: i/8 and 1/4 blasto- 

 meres give larvae with one and two costae respectively.^ In the un- 

 cleaved tgg of Beroe, there is a complete and uniform peripheral 

 layer of a clear substance which appears green by dark-field illu- 

 mination. By an elaborate series of changes, due to streaming move- 

 ments of the peripheral zone, and to alternation of more viscous 

 and less viscous phases in the general cytoplasm, the end of cleavage 

 sees this green substance lodged in the micromeres and forming 

 their entire contents, while none of it remains in the macromeres. 

 The micromeres give rise to the ectoderm, including the costae, - 

 and contain some materials, precociously chemo-differentiated 

 in the green substance, needed for costa-formation (figs. 49, 50). 



At the beginning of each cleavage division during the early 

 1 Fischel, 1898. 2 spek, 1926. 



