62 THE POPULAR SCIENCE MONTHLY. 



when first separated, are still naked, but they soon become surrounded 

 by a cell-membrane " (p. 14) ; and that in higher plants, as in the pol- 

 len of many Dicotyledons, " the contracting daughter-cells secrete cel- 

 lulose even duriug their separation " (p. 14). Here, then, in whatever 

 way we interpret it, the fact is that there quickly arises an outer layer 

 different from the contained matter. But the most significant evidence 

 is furnished by " the masses of protoplasm that escape into water from 

 the injured sacs of Vaucheria, which often instantly become rounded 

 into globular bodies," and of which the " hyaline protoplasm envelopes 

 the whole as a skin" (p. 41) which " is denser than the inner and more 

 watery substance " (p. 42). As in this case the protoplasm is but a 

 fragment, and as it is removed from the influence of the parent-cell, 

 this differentiating process can scarcely be regarded as anything more 

 than the effect of physico-chemical actions : a conclusion which is sup- 

 ported by the statement of Sachs that " not only every vacuole in a 

 solid protoplasmic body, but also every thread of protoplasm which 

 penetrates the sap-cavity, and finally the inner side of the protoplasm- 

 sac which encloses the sap-cavity, is also bounded by a skin " (p. 42). 

 If then " every portion of a protoplasmic body immediately surrounds 

 itself, when it becomes isolated, with such a skin," which is shown in 

 all cases to arise at the surface of contact with sap or water, this pri- 

 mary differentiation of outer from inner must be ascribed to the direct 

 action of the medium. Whether the coating thus initiated is secreted 

 by the protoplasm, or whether, as seems more likely, it results from 

 transformation of it, matters not to the argument. Either way the ac- 

 tion of the medium causes its formation ; and either way the many 

 varied and complex differentiations which developed cell- walls display, 

 must be considered as originating from those variations of this phys- 

 ically-generated covering which natural selection has taken advan- 

 tage of. 



The contained protoplasm of a vegetal cell, which has some self- 

 mobility and when liberated sometimes performs amoeba-like motions 

 for a time, may be regarded as an imprisoned amoeba ; and when we 

 pass from it to a free amoeba, which is one of the simplest types of 

 first animals, or Protozoa, we naturally meet with kindred phenomena. 

 The general trait which here concerns us, is that while its plastic or 

 semi-fluid sarcode goes on protruding, in irregular ways, now this and 

 now that part of its periphery, and again withdrawing into its interior 

 first one and then another of these temporary processes, perhaps with 

 some small portion of food attached, there is but an indistinct differ- 

 entiation of outer from inner (a fact shown by the frequent coalescence 

 of pseuopodia in Rhizopods) ; but that when it eventually becomes 

 quiescent, the surface becomes differentiated from the contents : the 

 passing into an encysted state, doubtless in large measure due to in- 

 herited proclivity, being furthered, and having probably been once 

 initiated, by the action of the medium. The connexion between con- 



