FrenzeVs Mesozoon Salioella. 471 



kinds of tlicm arc present, only the egt^-cclls, which enjoy 

 such favourable conditions as to virtually pass through in 

 their own ontogeny the whole unicellular phylogeny of the 

 species, and thereby to be able to transmit to their successors 

 the complete character of that form of life. 



Yet pretty frequently in the vegetable kingdom, but more 

 rarely in that of the animals, cases also indisputably occur 

 in which cells which had already adapted themselves to a 

 special function, and which we sliould therefore be inclined 

 to term working cells (" ArbcitszcUen ") in contrast to the 

 reproductive cells, under certain circumstances rejuvenate 

 themselves as it were, resume their virtual further develop- 

 ment, and consequently, when they have arrived at the 

 highest unicellular stage of their species, themselves become 

 reproductive cells. But if, in consequence of excessive 

 specialization or of the accumulation of aplasmatic cell- 

 products, they have forfeited their capacity to virtually attain 

 the highest unicellular stage, the daughter-cells which may 

 be produced from them will also be unable to arrive at anything 

 of the kind, and will never, even virtually, reach a higher 

 stage of development than their parent-cell. For tliis reason 

 the successors of already specialized tissue-cells can never do 

 anything else than at the utmost develop, multiply, or regene- 

 rate the same tissue ; and it is only in consequence of this 

 that the working cells can never produce from themselves a 

 new, independent, multicellular individual, similar to the 

 mother. 



Perhaps I am not mistaken if I consider that the theory of 

 morphogeny appears to be inapplicable to the Protozoa only 

 for the same reasons as those which are the cause of difficul- 

 ties in the interpretation of the embryological stages among 

 the Metazoa also — upon which, moreover, the differentiation 

 of the body-cells likewise depends; and (briefly to repeat 

 once more what has already been stated) these reasons them- 

 selves depend upon the fact that the different cells arrive at a 

 different grade of virtual development, the highest possible 

 stage of which is actually attained by the egg-cell alone: 

 then, remaining stationary at an earlier or later stage, they 

 display an organization which differs according to their con- 

 ditions, at the same time adapting themselves in one direction 

 and becoming far too much exhausted to be able to have a 

 further future. 



I will not, however, weigh every sentence of Frenzel's 

 article so precisely, although indeed we only w^eigh that which 

 appears to us to be worth weighing. Otherwise I might be 

 charged with fault-finding. 1 hope nevertheless that Frenzel 



