Frenzel’s Mesozoon Salinella. A471 
kinds of them are present, only the egg-cells, 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 should therefore be inclined 
to term working cells (“ Arbeitszellen”’) 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. or this 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 weigh that which 
appears to us to be worth weighing. Otherwise I might be 
charged with fault-finding. I hope nevertheless that Frenzel 
