INTRODUCTION. 5 



velopement 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 universal, would have "been made in 

 this extensive tribe of Nature's wondrous works ; so far, how^ 

 ever from this being the case, they have not as yet, so far 

 as I can learn, been noticed in any species of Alga; a 

 description of them, therefore, as they occur in two genera of 

 freshwater Conferva, Zygnema and Vesiculifera, cannot fail 

 to be of interest. 



In the first of these genera, Zygnema, their structure is 

 exceedingly complicated.* Each cytoblast is solitary, and * 

 usually occupies a central situation in each cell of a Zygnema. 

 It consists generally of two membranes, but sometimes 

 there are three ; the innermost of these being either circular 

 or elliptical, (the form varying with the species itself, as well 

 as its condition,) and presenting a nucleated appearance ; and 

 all are separated from each other by distinct intervals, which 

 are filled with fluid. The surface of the enclosed membrane 

 or membranes is smooth; while that of the external is ren- 

 dered irregular by the giving off of numerous tubular pro- 

 longations or radii, which terminate in the spiral threads 

 formed by mucus, endochrome, and large bright granules, , / 

 which I regard as the unfertilized zoospores. 



Wishing to have a corroboration of my views respecting 

 the structure of the cytoblastic organ described above, and also 

 to learn as much respecting its anatomy as possible, I for- 

 warded a specimen of Zygnema nitidum to that able and most 

 obliging observer, J. S. Bowerbank, Esq., whose opinion of 

 its structure exactly coincides with my own, that gentleman 

 having in particular 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 

 satisfaction one of the offices which it is destined to discharge, 



o * 



viz., that of a laboratory or stomachy in which the materials 



necessary for the growth and vitality of the cell and its con* 



tents are received and digested, and from which they are 



* See Plate 17. Jig. 1, 2 3, 



B 3 



