THE GERM-PLASM THEORY 407 



Morgan, Conklin, Wilson, Crampton, and Fischel— not only leave 

 the essential part of the germ-plasm theory untouched, but rather 

 strengthen than endanger its more subordinate points, such as the 

 assumption of a segregation of the components of the germ-plasm in 

 the course of ontogenesis. 



As to the fundamental ideas expressed in the theory, I have 

 already shown that these remain unaltered, even if we do not assume 

 a disintegration or segregation of the germ-plasm, but think of all 

 the developing cells as equipped with the complete germ-plasm. In 

 that case the determinants would be liberated to activity solely by 

 specific stimuli. But in regard to the assumption of disintegration, 

 it must be noted that the facts cited relative to the sea-urchin's ova 

 do not by any means hold true of the eggs of all animals. 



In various animal types each of the first two segmentation-cells, 

 when separated from its neighbour, produces only a half-embryo, and 

 any one of the first four cells a quarter-embryo. This 'fractional 

 embryo ' is, however, in some cases able later to develop into a whole 

 embryo (to ' postgenerate ' itself, as W. Roux says). The isolated 

 blastomere shows, to begin with, an activity of only a half of the 

 primary constituents of the animal, as was first established by 

 W. Roux and maintained conclusively, in spite of many attacks, until 

 it was established beyond doubt by the detailed corroboratory in- 

 vestigations of Endres. The secondary completion of the embryo, 

 which, however, is still disputed, must be regarded as a regeneration, 

 and, to explain it, a co-operation of the complete but not yet wholly 

 active germ-plasm in both segmentation- cells must therefore be 

 assumed. 



It would carry us too far if I were to deal in detail even with 

 the most important of the numerous facts that the last decade has 

 brought to light ; I shall restrict myself to the most essential. 



That isolated segmentation-cells have the capacity of developing 

 into embryos which are complete but correspondingly smaller in size 

 has been demonstrated in animals of various groups, though it does 

 not seem to go to the same length in all. In the Medusas we find 

 that not only one of the first two, but one of the first four, eight, and 

 even sixteen segmentation-cells may develop a whole larva when 

 isolated (Zoja). In the sea-urchin at least any one of the first eight 

 blastomeres may do so. And Driesch's experiments in cutting up the 

 young larva? at the blastula-stage (a single-layered ball of cells) leads 

 us to assume that each of these cells still possesses the complete germ- 

 plasm. Beyond that stage, however, the primary constituents 

 obviously divide into those of the ectoderm and those of the endo- 



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