THE CELL-THEORY 265 



a " cell-force," of whose existence there is on other grounds no evidence 

 whatever ? We must answer in the negative. For us the primarily- 

 cellular structure of plants and animals is simply a fact in the history 

 of their histological development — a histologically necessary stage, if 

 one may so call it, which has no more causal connexion with that 

 which follows it, than the equally puzzling morphological necessity for 

 the existence of a chorda dorsalis or of Wolffian bodies has with the 

 •development of the true vertebrae or of the true kidneys. 



If this be true, we might expect, as we find, that the differentiation 

 of the germinal disc, for instance, into a primitive groove and lateral 

 portions — the first stage of development in the embryo of all vetebrate 

 animals — does not occur in mollusks ; as we find, again, that the 

 differentiation of the embryo into plumula and cotyledons which occurs 

 in a great number of plants is absent in others ; so if, like these, the 

 histological differentiation into cells have no necessary causal connexion 

 with the action of the vital forces, but be merely a genetic state, we 

 may expect to meet with cases in which it does not occur. Such, in 

 fact, are the so-called unicellular plants and animals — organisms which 

 often exhibit no small complexity of external form, but present no 

 internal histological differentiation. In the genus Caulerpa we have 

 an Alga presenting apparent leaves, stems, and roots, and yet which, 

 according to Nageli, consists of a single cell — that is, is not composed 

 of cells at all. The Vorticellse furnish us with examples of animals 

 provided with a distinct oesophagus, a muscular pedicle, &c., and yet 

 in which no further histological differentiation can be made out. As 

 Wolffs says — 



" The latter (Roesel's Proteus) has no structure, no determinate 

 figure, and even the indeterminate figure that it has at any given time 

 does not remain the same, but alters continually. We can, in fact, 

 regard all these plants and animals as little else than living or vegeta- 

 ting matters — hardly as organized bodies. 



" § 74. However, all these plants and animals nourish themselves, 

 vegetate, and propagate their species, just as well and as easily as the 

 most artificial pieces of mechanism to be met with in the vegetable or 

 animal kingdom." 



It is true, indeed, that the difficulty with regard to these organisms 

 has been evaded by calling them " unicellular " — by supposing them to 

 be merely enlarged and modified simple cells ; but does not the phrase 

 an " unicellular organism " involve a contradiction for the cell-theory? 

 In the terms of the cell-theory, is not the cell supposed to be an 



' ' Von der wesentlichen Kraft,' p. 40. 



