6 TO WILSON. [Vol. VIII. 



upon the extent to which its idioplasm has become modified; 

 second, (as Driesch has suggested) upon new mechanical con- 

 ditions due to the diminution of size or change of form, such 

 as greater surface-tensions caused by diminished radius of 

 curvature, and the like. In the 2-celled stage the modification 

 of the idioplasm is in AmpJiioxus very slight, and the isolated 

 blastomere generally reverts at once to the condition of the 

 original ovum, tJwugh soinethnes varying from it to some degree. 

 In the 4-cened stage the idioplasm is further modified, though 

 not to such a degree as to prevent its return to the original 

 condition. By the 8-celled stage it is incapable of returning to 

 the original state, and the normal type of cleavage is no longer 

 repeated; and so in a still greater degree in later stages. We 

 can thus understand how it comes to pass that the differentiated 

 germ-layers of later stages lose in general their power to re- 

 generate the other germ-layers. The specialization of the 

 idioplasm, like that of the cell as a whole, appears to be a 

 cumulative process that results in a more and more fixed mode 

 of action. Hence its gradual loss of power to return under 

 changed conditions to the state of original germ-plasm, though 

 it may contain all of the elements of germ-plasm. The inde- 

 pendent, self-determining power of the cell ( " Selbstdifferen- 

 zierung" of Roux), therefore, steadily increases as the cleavage 

 advances. In other words : the otitogeny assumes moi'e and more 

 of the character of a mosaic-work as it goes forwards, hi the 

 earlier stages the niorpJio logical value of a cell may be determined 

 by its location. In later stages this is less strictly true and in 

 the end the cell may become more or less cotnpletely i^idependeiit 

 of its location, its substance having become finally and pcr- 

 vianently changed. The power of regeneration, often existing 

 in late stages or in the adult, may be regarded as a special 

 adaptation such that the idioplasm in some of the cells retains 

 in a greater or less degree the plasticity of earlier stages — why 

 or how, it is at present impossible to say further than that the 

 power to do so is in some way the outcome of a specific 

 peculiarity of the original germ-plasm. 



How, now, shall we apply this interpretation to the case 

 of Echinus, in which the isolated J^ blastomere pursues for a 



