Ill DIFFERENTIATION 77 



replace one another in regeneration. The power of giving rise 

 to whole structures therefore only disappears because the 

 cleavage takes a certain course, and, were the divisions to con- 

 tinue in one kind of direction, might conceivably be indefinitely 

 retained, just as the compressed egg of Nereis, or the egg 

 of Cerebratulus in calcium-free sea-water produces octettes 

 instead of quartettes of micromeres. 



The limitation of potentialities seen in later stages is due 

 again to the original regional differences in the egg-material. 

 Thus endoderm and ectoderm can as little replace one another 

 in a Sea-urchin gastrula as in a regenerating Hydra, or in 

 a Hydra that has been turned inside out (the experiment of 

 the Abbe* Trembley is well known). It is stated indeed 

 (Driesch) that any fragment of a blastula of a Sea-urchin can 

 give rise to as normal a larva as any other, but this needs 

 re-examination. 



On the other hand, organs normally formed by one part of 

 the embryo can be developed by another. The posterior half- 

 gastrula of a Nemertine develops an apical organ, that of 

 a Sea-urchin an apical organ, a stomodaeum, and a tripartite 

 gut; and here we are in the presence of the phenomenon 

 which is characteristic of all regeneration, the develop- 

 ment of a whole structure from a part of a differentiated 

 structure. 



When a (meridional) half- or quarter-blastomere of a polar 

 egg is differentiated into a complete larva, that appears to be 

 due to its possessing a share of each necessary substance ; but 

 when a worm regenerates a head, or a newt the lens of the 

 eye, or a crayfish a limb, this simple explanation will not 

 avail. Regeneration is not within our present scope, but the 

 behaviour of certain blastomeres when isolated, raises precisely 

 the same question. 



We have seen that in the Frog not only from the four 

 vegetative but also from the four animal cells a blastopore 

 and archenteron can be developed. Similarly in the Sea- 

 urchin, not only a vegetative, but also an animal, blastomere 

 can gastrulate, though the latter does so neither as well nor 

 as frequently as the former. In other words, a structure can 



