Ch. XII] IXTERPRETATIOXS AXD COXCLUSIOXS 133 



largely absent, either because the blastomeres cannot be brought 

 into a new whole, or because the protoplasm is so fixed, so stiff, 

 that it cannot readily rearrange itself. If from either of these 

 conditions, or from some other, the blastomeres are not capable 

 of rearrangement or reconstruction, an imperfect embryo re- 

 sults. 



How far does the totipotence of the blastomeres reach? Does 

 it end with the two-cell, four-cell, or later stages of cleavage ? 

 Probably this varies in different eggs. The one-fourth blasto- 

 mere of Echinus can form a perfect embryo, and even the one- 

 eighth blastomere may develop into a gastrula. The same is 

 true for the egg of Amphioxus. For the frog it is not yet 

 possible to say where the limit lies. In this connection the 

 following facts are of importance. The isolated blastomere 

 of the sea-urchin's egg runs through the same number of divi- 

 sions that it would have done had it remained in contact with 

 its fellows.^ Hence the half-embryo has only one-half the 

 number of cells of the normal embryo, and the one-fourth em- 

 bryo has only about one-fourth the full number. This seems 

 to give, in part, an explanation of the statement made above, 

 viz., that the one-half embryo develops further than the one- 

 fourth, and the latter further than the one-eighth, since the 

 smaller the isolated blastomere the fewer are the cells it pro- 

 duces from which the embryo is formed. The lack of power of 

 development of the small isolated blastomere is not, therefore, 

 dependent on its differentiation. This is also shown by the fol- 

 lowing experiment. In the blastula-stage of the sea-urchin's 

 egg, pieces may be cut or shaken from the blastula-wall, and, 

 if large enough, they develop into small larvae. Here also we 

 find that the large pieces can go further in the ontogeny than 

 the smaller pieces, probably owing to the presence of a sufficient 

 number of cells or of sufficient material to form the necessary 

 organs of the embryo. ^ 



If the early blastomeres are totipotent, what brings about the 

 later differentiation of these cells ? There are sufficient reasons. 



1 Morgan ('95). 



2 The same experiment cannot be made on the frog's blastula, because, if 

 cut, the pieces immediately disintegrate. 



