THE FACTORS OP ORGANIC EVOLUTION. 51 



jugated protoplasmic body clothes itself with a cell-wall &quot; 

 (p. 10) -, that in &quot; the spores of Mosses and Vascular Crypto 

 gams &quot; and in &quot; the pollen of Phanerogams &quot; . . . &quot; the 

 protoplasmic body of the mother-cell breaks up into four 

 lumps, which quickly round themselves off and contract, and 

 become enveloped by a cell-membrane only after complete 

 separation&quot; (p. 13); that in the Equisetaceze &quot;the young 

 spores, when first separated, are still naked, but they soon 

 become surrounded by a cell-membrane &quot; (p. 14) ; and that 

 in higher plants, as in the pollen of many Dicotyledons, 

 &quot;the contracting daughter-cells secrete cellulose even 

 during their separation&quot; (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 &quot;the masses of 

 protoplasm that escape into water from the injured sacs 

 of Vaucheria, which often instantly become rounded into 

 globular bodies,&quot; and of which the &quot;hyaline protoplasm 

 envelopes the whole as a skin&quot; (p. 41) which &quot;is denser than 

 the inner and more watery substance &quot; (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 supported by the statement of Sachs that &quot;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&quot; (p. 42). If 

 then &quot; every portion of a protoplasmic body immediately 

 surrounds itself, when it becomes isolated, with such a 

 skin,&quot; which is shown in all cases to arise at the surface of 

 contact with sap or water, this primary 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 



