of the Freshivater Alga. 21 



cytoblasts, which exercise an influence so mysterious on the de- 

 velopment of cells, and whose presence in cellular structure is so 

 constant as to lead to the suspicion that the association of the 

 two organisms is uni^•ersal, would have been made in this exten- 

 sive tribe of Nature's exhaustless works ; so far however from this 

 being the case, they have not as yet, from what I can learn, been 

 noticed in any species of Alga ; a description of them therefore, as 

 they occur in two genera of freshwater Confervas, Zygnema and 

 Vesiculifera, cannot fail to be of interest. 



In the first of these genera, Zygnema, their structm-e is exceed- 

 ingly complicated. Each cytoblast is sohtary, and usually occu- 

 pies a central situation in each cell of a Zygnema. It consists 

 generally of two membranes, but sometimes there arc three ; the 

 innermost of these being either circular or elliptical (the form 

 varying with the species itself as well as its condition) and pre- 

 senting a nucleated appearance, and all are separated from each 

 other by distinct intervals which are filled with fluid. The sm-- 

 face of the inclosed membrane or membranes is smooth, while 

 that of the external is rendered irregular by the giving off" of nu- 

 merous tubular prolongations or radii which terminate in the 

 spiral threads formed by mucous endochrome and large bright 

 gi-anules, which I regard as the unfertilized zoospores (PI. I. 

 fig.l). . 



Wishing to have a corroboration of my views respectnig the 

 structm-e of the cytoblastic organ described above, and also to 

 learn as much respecting its anatomy as possible, I forwarded a 

 specimen of Zygnema nitidum to that able and most obliging ob- 

 server J. S. Bowerbank, Esq., whose opinion of its structure ex- 

 actly coincides -^-ith my ovm, that gentleman ha^dng in particidar 

 satisfied himself of the tubular nature of the prolongations sent 

 off" by the external membrane, and of their termination in the 

 spiral threads. 



The structure of this curious organ explains with apparent sa- 

 tisfaction one of the offices which it is destined to discharge, viz. 

 that of a laboratory or stomach, in which the materials necessary 

 for the growth and vitality of the cell and its contents are received 

 and digested, and from which they are conveyed by means of the 

 tubular radii to those organs by which the materials are to be as- 

 similated. 



The cytoblast, therefore, is at fii'st fixed in the centre of the cell 

 by the prolongations which proceed from it ; but it happens, that 

 at a certain epoch these radii disappear, and then the cytoblast 

 floats freely within the cavity of the cell ; the disappearance of the 

 rays, the cessation of the gro^A-th of the cells, and the assumption 

 of the characters of reproduction being almost contemporaneous, 

 or, at any rate, events immediately consecutive on each other, and 



