INTRODUCTION. 7 



productions ; while in other divisions of the same tribe, which 

 could not be supposed to differ fundamentally from the 

 former, no such phenomenon has hitherto been recognized ; 

 by shewing that this combination is not an essential to the 

 perpetuation of the species : and thirdly, it explains the per- 

 manence of species which have perished before union of the 

 endochrome and formation of spores have taken place. 



I have detected cytoblasts in numerous Zygnemata, but 

 the best species in which to examine them are the larger 

 kinds, such as Zygnema maximum, Z. nitidum, and Z. belle. 

 Of the genus Vesiculifera, I have also found it in several 

 species : they cannot always be seen in these, owing to the 

 cells not being so transparent. I doubt not, however, but 

 that they are general in it, as well as other genera of Alga, 

 whether marine or freshwater. In this genus it is but a 

 simple vesicle ; at least, I have never observed it in any other 

 state. (See Plate 17. Jig. 6.) 



The Rev. M. J. Berkeley has kindly favoured me with an 

 abstract of a paper by Hugo Mohl on the genus Anthoceros,, 

 published in 1839, and inserted in " Linna3a," vol. xiii. p. 273., 

 in the cells of which an organ occurs bearing a considerable 

 external resemblance to the radiated structure met with in 

 the cells of Zygnema. 



The following is a brief outline of the mode of formation 

 of this structure in the genus Anthoceros. When an imma- 

 ture cell of one of the species of this genus is examined, a 

 portion of its interior is seen to be occupied by a layer of 

 green granules, through which may be seen a cytoblast, the 

 other portion of the cell being colourless. Treated with 

 iodine, the layer formed by green granules, as also the colour- 

 less part of the cell, becomes yellow, showing that the whole 

 is really lined with a sort of quasi membrane. Gradually the 

 green layer becomes concentrated into two masses, which 

 commence to advance more and more towards the middle of 

 the cells, and the edges of these masses spreading in various 

 degrees over the inner wall of the cell, leave intervals of 

 various sizes, which give to them a cellular appearance. 

 " The nucleus, or cytoblast," Mohl observes, " has no part in 



B 4 



