UNICELLULAR PLANTS. 261 



appears to be a process very nearly allied to that which 

 takes place in Protococcus, if it be not identical with it. 

 The red substance of Prot. pluvialis is not always of an 

 oily aspect ; it only becomes so in more advanced age. 

 And according to Cohn's researches, this oily material is 

 much more generally distributed than has been supposed, 

 among the lower Algae; occurring in many true brown 

 spores, such as of (Edogonium, Spirogyra, Vaucheria, &c. 



When still or motile cells of Protococcus are brought in 

 contact with a very weak solution of iodine, they become 

 internally, in most parts, of an intense violet or blue colour. 

 With respect to the solid constituents of the Protococcus 

 cell contents, they may be distinguished into chlorophyll 

 vesicles, colourless or green particles, amylaceous granules, 

 and nucleus. The motile form of Protococcus consists, 

 as it were, of two cells, one within the other, both of which, 

 however, differ essentially from the common vegetable cell : 

 the external having a true cell-membrane and fluid con- 

 tents j the other, or internal one, with denser, muco-gela- 

 tinous coloured contents, but without a true cell-wall. 

 Cohn called the external transparent vesicle the " enve- 

 loping " cell," and the internal coloured one the " primor- 

 dial cell." The term "primordial sac, or utricle, " can 

 only be applied to its peripheral layer, and not to that 

 together with the contents. 



The form of Protococcus (fig. 144) presents a perfect 

 analogy between the primordial cell and the nucleus of 

 the common plant-cell. The filaments which proceed 

 from the central mass to the peripheric cell-wall, are 

 tubular, giving passage to the red molecules from the 

 central mass. These filaments, however, which proceed 

 from the outer wall of the primordial cell towards the 

 inner surface of the enveloping cell, correspond morpho- 

 logically to the so-termed mucous filaments by which the 

 cytoblasts are commonly retained in the centre of their 

 cells. That they also correspond chemically with these, 

 is proved by the fact that they are rendered more distinct 

 by iodine, and that they can be made to retract by means 

 of reagents ; and in fact they exhibit, in the course 

 of development, peculiarities which characterise them as 

 consisting of protoplasm. 



