IV. i INITIAL STRUCTURE OF THE GERM 



187 



the formation of the micromeres suppressed in Strongylocentro- 

 tus and a normal larva still result. The relation, however, of the 

 egg-axis, as determined by the pigment-ring, to the symmetry 

 of the larva remains the same as in the undisturbed egg. 



Increased temperature, violent shaking, dilute sea-water, and 

 calcium-free sea-water are all means by which Driesch has suc- 

 ceeded in disarranging the blastomeres of Echinus in various ways 

 (Fig. 94). Thus when the temperature was raised from 19 C. (the 



FIG. 95. Variations in the segmentation of Echinus microtuberculatus 

 produced by dilution of the sea-water, a, tetrahedral four-cell stage ; 

 b, eight cells, three premature micromeres ; c, eight cells, two precocious 

 micromeres ; d, the same egg after the next division, the precocious 

 micromeres have divided unequally, two normal micromeres have been 

 formed. t After Driesch, 1895.) 



normal) to 31C. the first two (not subsequent) blastomeres 

 separated, though they sometimes reunited. The next division 

 was normal, but with the next phase one or two blastomeres 

 divided in a direction perpendicular to that of the others, or the 

 direction of division was different in each. In the following 

 stage the micromeres were wholly or partly suppressed. So also in 

 dilute sea- water (20% fresh water) the third division was unequal 



