470 Prof. S. Apc4tl.y on 



of development, can be rediscovered in ontogeny also, and 

 indeed both among the Protozoa and the Metazoa as well. 

 The ontogeny of a Metazoon individual does not commence 

 with the stage of the fertilized or unfertilized egg-cell in 

 process of division from which the Metazoon is built up ; 

 but the individual itself, which is represented by the mature 

 egg-cell, has a past of its own which was possibly of great 

 length, and which commenced with the non-nucleate stage 

 within that germ-cell, from whose division into two it imme- 

 diately proceeded as an unripe egg-cell. 



I am unacquainted with any facts — it may be that my 

 knowledge is insufficient for the purpose — which would 

 render the theory of morphogeny inapplicable to the Protozoa, 

 especially as, between the visible stages of the development 

 of their organization, there may be others which are invisible. 

 Development may even attain the highest stage of unicellular 

 existence without evolving further organization ; for it con- 

 sists in a series of transformations of the properties of the 

 Protoblast, in imitation of the sequence of events in the 

 phylogeny, wherein each arrangement of organs corresponding 

 to the particular stage of development is only potentially 

 combined with the succession of these transformations — that 

 is, the latter includes only the capacity to produce such organ- 

 ization should circumstances require it. 



In this manner it seems to me that the egg-cell ontogeneti- 

 cally arrives at the highest stage of unicellular existence 

 which has been present in the phylogeny of that form of life; 

 and all its daughter-cells and subsequent descendants, the 

 constituents of the Metazoon body, have the capacity to reach 

 the same stage, and must endeavour to reach it by the same 

 way, starting from the stage of the non-nucleate Protoblast. 

 The rapidity of the development varies according to the con- 

 ditions under which the particular cell commences and con- 

 tinues to maintain its individual life. The greater portion of 

 the cells of the Metazoon body, however, owing to the con- 

 ditions which obtain at an earlier or later stage of the onto- 

 geny of the latter, is compelled actually to develop the 

 organization which belongs to this particular stage, although 

 it may not be exhibited hy other cells of the body. Those 

 cells which, at whatever stage, really have to develop their 

 organization, are hindered in their potential further develop- 

 ment owing to immediate one-sided adaptation, are usually 

 enfeebled in consequence of the performance of special func- 

 tions, and never attain the highest stage of development, to 

 which at their cnigin they were, so to speak, historically 

 predestinated. It is only the reproductive cells, or, if two 



