408 THE EVOLUTION THEORY 



derm, for the subsequent two-layered stage in the sea-urchin's de- 

 velopment, the gastrula, does not complete itself if it be artificially 

 divided into fragments which consist only of cells from the outer, or 

 only of cells from the inner layer. In corroboration of this experi- 

 ment made by Barfurth, Samassa was able to demonstrate in regard 

 to the egg of the frog that, even after the third division of the ovum, 

 the segmentation-cells are so different from each other in respect of 

 their primary constituents that they were not able to replace each 

 other mutually. When this investigator killed the ectoderm-cells 

 alone by means of an induction current, or the endoderm-cells alone, 

 the dead half could not be replaced by the half which remained alive, 

 and the whole ovum perished. 



If these facts may be adduced in favour of a separation of the 

 primary constituents at an earlier or later stage, we find even stronger 

 proofs among the Ctenophores, Gastropods, Bivalves, and Annelids. In 

 the last-named group Wilson has shown it to be probable that develop- 

 ment is really a ' mosaic work,' as Eoux and I had assumed. The older 

 observations made by Chun at an earlier date on the Ctenophora, 

 and the more recent experiments of Fischel on the same animals, 

 prove the same thing for this group. In this case complete larvae are 

 easily distinguished from mere ' partial developments ' by the number 

 of the characteristic ' ciliated meridional rows ' or ribs, which extend 

 from one pole of the larva to another. In the complete larva there 

 are eight of these, but in larvae from one of the first two blastomeres 

 (isolated) there are only four, and in those which have arisen from 

 one of the first four blastomeres there are only two. If an ovum at the 

 eight-cell stage can be successfully divided into separate blastomeres, 

 each of these will form an ' eighth larva,' always with only one 

 ciliated rib. Even in the succeeding sixteen-cell stage it could still be 

 demonstrated that the substance responsible for the formation of the 

 ribs only lies in particular places and always sufiices only for eight 

 ribs. The sixteen-cell stage consists of eight large cells and eight 

 small ones, the ' macromeres ' and the ' micromeres ' ; if an ovum at 

 this stage be cut so that one piece contains five macromeres and five 

 micromeres, a partial la^a will develop which possesses only five ribs, 

 while the larva from the other portion will have only three. But the 

 localizing of the rib-determinants can be followed still further, for in 

 larvae in which individual micromeres have been displaced from their 

 normal position there is a correlated displacement of the corre- 

 sponding ribs, and a dislocation of their ciliated comb-plates. The 

 determinants of the ribs must therefore lie in the micromeres, and we 

 must conclude that at the antecedent divisionthey were only imparted 



