OF THE PHYTOZOA. 113 



represents a true cell-membrane, enclosing between itself and the coloured 

 substance a colourless aqueous fluid, probably pure or nearly pure water." 

 And in the subsequent considerations of this structiu-e, Cohn appears to arrive 

 at the conviction that the internal coloiu-ed body generally spoken of as the 

 cell, the actual unicellular organism, represents the nucleus of a cell, of which 

 the periphery of the mucous envelope is the boundary. In this interpretation 

 of the natiu'e of the mucilaginous envelope, Prof. Williamson concm\s. Indeed 

 this accurate observer proceeds, further, to show that there is in the case of 

 Volvox a true enclosing delicate membrane to each cell, and that the hexa- 

 gonal form is owing to the mutual pressui^e of the aggregated cells (PI. 

 XX. 38). In aggregate forms, such as Volvox, Goniiun, Fandorlna, &c., an 

 additional common external membrane would seem to be tlu^own out, to unite 

 together into one symmetrical whole the various members of the colony. 

 Perhaps it should be rather called a pellicle than a membrane, seeing that its 

 independent existence as a separable structm^e cannot be demonstrated : yet 

 it has a power of resistance ; for when external force is applied to a globe of 

 Volvox, the surface, though at first depressed, presently recovers itself by an 

 innate elasticity ; and in the case of Pandorina it seems so resistant and firm 

 that it does not indent on pressure (XIX. 61 ). 



Cell-coxtexts. — The fluid distending the mucilaginous envelope around 

 most Phytozoa, in one or other stage of being, is, according to Cohn, as above 

 noticed, probably pure water. This opinion Prof. Williamson does not enter- 

 tain; for he says (J. M. S. 1853, p. bb) "it is apparently mucilage. In a 

 preparation in which a nimiber of these objects [of Volvox'] are mounted in 

 dilute alcohol, this gummy matter has changed to a bro-svn coloiu^ and refused 

 to mingle ^vith the alcohol, as would be the case supposing it to be mucila- 

 ginous. This proves that it is a true secretion from the organism, and not 



merely water absorbed by endosmosis The secretion itself is, perhaps, little 



more than a diluted condition of the same gum as that which is more or less 

 completely converted into cellulose in the various investing membranes." 



The central globule, or the whole recognized organism where a mucilaginous 

 envelope is not present, consists of a mass of protoplasm. At fii'st it is homo- 

 geneous and without colour ; subsequently it becomes generally coloured and 

 granular ; but very shortly the included matters gather together into a sort of 

 layer subjacent to the surface, and leave the central part clear, sometimes so 

 completely so that it assumes the appearance of a vacuole. This substance 

 moreover has the property of contractility inherent in it, and would seem, in 

 all essential circumstances, homologous with the simple contractile matter — 

 the sarcode of animalcules. Like the latter, it may hollow itself out into 

 vacuoles at any part ; and such, says Cohn {R. S. p. 535), " are present in all 

 yoimg cells, and play a considerable part in cell-division and the sap- currents." 

 The property of contractility is singularly displayed in the case of the actively 

 moving zoospores or sporozoids of the Algae, and in the motile form of Proto- 

 coccus, — {. e. in every instance where, from the absence of more or less inelastic 

 membranes, it can exhibit itself. The vacuoles of the protoplasm occur in 

 varying numbers, and change or disappear from time to time : within they 

 contain an aqueous fluid. 



The contractile protoplasm is itself colourless ; yet, except in the earliest 

 stages of development, it partakes of a green or a red colour, or of both these 

 coloui's together, save in one spot, which in oblong forms is situated at one 

 end, and in the projection or beak (proboscis, Ehr., or rostellum) extending 

 from the anterior extremity. " It appears," says Cohn {R. S. p. 536), " as 

 a dehcate, almost imperceptible layer constituting the outer boundary of the 

 coloured primordial ceU, the peripheiy of which then becomes shai7)ly de- 



