68 CLEAVAGE II 



included the polar lobe, which other experiments have shown 

 to contain the material necessary for the formation of the 

 sense-organ and post-trochal region, develops into a complete 

 larva. 



In these cases a partial segmentation is followed by a total 

 development. 



The converse is seen in the total cleavage, at low tem- 

 peratures, of blastomeres of Ilyanassa which are still unable 

 to form whole larvae, and again in the complete segmentation 

 of the Ctenophor egg from which a piece of the vegetative 

 hemisphere has been removed, though the larva that is 

 derived from such a fragment is defective (Driesch and 

 Morgan). 



Thirdly, it is well known that in the spirally segmenting 

 eggs of Nemertines, Turbellarians, Molluscs, and Annelids, 

 it is possible to trace the several organs and tissues of the 

 embryonic body each back to an origin in some particular 

 cell or group of cells in the cleavage system, and that as 

 a general rule homologous organs, say the prototroch or the 

 mesoderm, arise from cells which occupy identical positions 

 and come into being by the same sequence of divisions in 

 all cases. Thus the mesoderm usually is derived from the 

 cell 4 d ; 2 d, the first somatoblast, gives rise to the ectoderm 

 of the ventral plate ; the first quartette of micromeres is 

 ectodermal and so forth. But there is no necessity that 

 identical cells or cell-groups should have the same destiny, 

 and there are many exceptions known to the general rule. 

 One instance will suffice. 



In the earthworm the nephridia, being derived from the 

 ectoderm of the ventral plate can be traced back to 2 d, while 

 in another Annelid, the leech, the same organs are mesodermal 

 in origin and spring from 4 d. These need cause no per- 

 plexity, for, as we shall see more fully in a moment, the 

 organs depend for their development upon the presence of 

 some factor in the cytoplasm, but this factor (a material 

 factor) need not be present in that position in which the 

 organ to which it 'is appropriate will arise. Thus the factor 

 which conditions the differentiation of nephridia, being in 



