136 CONJUGATED. 



comes everted and protruded into tlie cavity of the other. 

 (PI. XYII. fig. 5.) 



" The cause of this protrusion, and the reason why it only 

 occurs at the precise period of the reproduction of the cells, 

 are easily accounted for, and both arise from imequal internal 

 pressure of the contiguous cells on each other, which ine- 

 quality of pressure is produced by the emission of the endo- 

 chrome of one cell into a neighbouring cell either in the same 

 or different filaments ; thus, when a cell has discharged its 

 contents, its cavity is empty, and no resistance can be offered 

 by it to the protrusion of the inverted portion of the adjacent 

 cell or cells, replete as it or they may be with fluid and en- 

 dochrome. This explanation applies likewise to the fact, that 

 when a number of cells have either emptied themselves of 

 their contents, or have been the recipients of those of other 

 cells at the same time, no eversion takes place, for in this case 

 there is no inequality of internal pressure. 



" But while a correct exposition may be given of the cause 

 of this protrusion and intromission, it is not so easy to oflTer 

 a satisfactory explanation of the purpose to be attained by it. 

 The eversion, doubtless, assists in effecting the dislocation of 

 the cells, and thus, reproduction being perfected, hastening the 

 destruction of the species and dispersion of the spores ; pro- 

 cesses, w^hlch, from the greater length of the cells and con- 

 sequent continuity of the enveloping sheath, would possibly 

 occupy, were it not for some special provision of the nature 

 indicated, a much longer time than in the short -celled species. 

 A subordinate and not unimportant use of this provision is, the 

 assistance whicli it aflfords in the determination of alHed species. 



« It is remarkable that no similar conformation presents 

 itself to our notice in the genera Tyndaridea and Mougeotia, 

 so closely allied to Zygnema, for in these the cells invariably 

 terminate by plane surfaces, which, however, may be either 

 everted or inverted to a slight extent. 



" This peculiar formation of the cells of some Zygnemata 

 w^as first noticed by me in the spring of 1842, but its true 

 uatiu-e only became apparent to me in the early portion of 

 the present year. When A^ewed tln-ough a low power of the 

 microscope, and in a Zygnema whose filaments are as yet 



