Regeneration 185 



the eggs of Echinoderms. We stated in the previous 

 chapter that Driesch produced artificially larvae of sea 

 urchins of one-eighth, one-fourth, and one-half their 

 normal size by isolating a single cleavage cell in one of 

 the first stages of segmentation of the fertilized sea- 

 urchin egg. He counted in each of the dwarf gastrulae 

 resulting from these partial eggs the number of mesen- 

 chyme cells and found that the larvae from a one-half 

 blastomere possessed only one-half, those from a one- 

 fourth blastomere only one-fourth, and those from a 

 one-eighth blastomere only one-eighth of the number of 

 cells which a normal larva developing from a whole egg 

 possessed. Moreover, he could show that when two 

 eggs were caused to fuse so as to produce a single larva 

 of double size, the gastrulae of such larvae had twice the 

 number of mesenchyme cells. Driesch drew the con- 

 clusion from his observations that each morphogenetic 

 process in an egg reaches its natural end when the 

 cells formed in the process have reached their final 

 size. 



Since each daughter nucleus of a dividing blastomere 

 has the same number of chromosomes as the original 

 nucleus of the egg, it is clear that in a normally fertilized 

 egg each nucleus has twice the mass of chromosomes 

 that is contained in the nucleus of a merogonic egg, i. e., 

 an enucleated fragment of protoplasm into which a 

 spermatozoon has entered and which is able to develop. 

 Such a fragment has only the sperm nucleus. This 



