DIFFERENTIATION, HEREDITY, SEX 



285 



remaining one, if undisturbed, develops into a half-embryo; 

 but if the egg is inverted after the injury of one blastomere, 

 then during the consequent rearrangement of the substances of 

 the uninjured cell, through the action of gravity, the organi- 

 zation is restored to the normal and a small normal embryo 

 subsequently develops. This shows that the uninjured half of 



FIG. 136. Fusion of Echinoderm larvae. A, B. Sphcerechinus. After Driesch. 

 C, D. Psammechinus miliaris. After Garbowski. A. Normal gastrula. B. 

 Single gastrula derived from the fusion of two normal blastulae, showing single, 

 large gut and doubled spicule. C. Normal stage of thirty-two-cells. D. Organ- 

 ism formed by the coalescence of parts of two organisms in different stages. The 

 cells ruled obliquely were part of an eight-cell stage, stained intravitally with 

 neutral red. The remaining cells were part of a normal thirty-two-cell stage. 

 In both C and D the stippling marks the cells derived from the vegetative half 

 of the egg. 



the egg does possess the potentiality of developing as a com- 

 plete egg. 



Or second, the contrast between the two extreme cases 

 mentioned may mean that localization results from a progres- 

 sive process of true development. Of course, in all organisms, 

 sooner or later, groups of cells become specifically differentiated 

 as particular tissues and organs or parts of organs. And 

 similarly there comes a tune in the history of any cell group 



