INTRODUCTION. 



the conjunction of four individuals for that purpose ; but if 

 the fronds were binate, it must follow that the cooperation of 

 four individuals would be necessary for the continuation of 

 the species. In Didymoprium Grevillii the reproductive body 

 is contained within one of the coupling joints ; so that if they 

 were binate we should have the contents of two cells passing 

 into two other cells, the contents of all four uniting into one 

 body, two of the cells at all events forming a single chamber. 

 It may indeed be suggested that the joint in Didymoprium 

 differs from the frond in some of the other genera ; but such 

 a supposition is utterly untenable, and will never be advanced 

 by any one acquainted with these objects. In both the 

 endochrome is divided into two by a pale transverse band 

 marking the junction of the valves ; and here they in both 

 eventually open, and permit the escape of their contents. In 

 the one case, as in the other, the coupling bodies alike com- 

 municate at this point, and the entire process is essentially 

 the same. I will now presume that I have proved that the 

 bipartite Desmidiese are truly cells more or less constricted, 

 and in the following details I shall so designate them. 



In the Desmidiese the multiphcation of the cells by repeated 

 transverse division is full of interest, both on account of the 

 remarkable manner in which it takes place, and because it 

 unfolds, as I beUeve, the nature of the process in other fami- 

 lies, and furnishes a valuable addition to our knowledge of 

 their structure and physiology. 



The compressed and deeply constricted cells of Euastrum 

 offer most favourable opportunities for ascertaining the man- 

 ner of the division ; for although the frond is really a single 

 cell, yet this cell in all its stages appears like two, the seg- 

 ments being always distinct, even from the commencement. 

 As the connecting portion is so small, and necessarily pro- 

 duces the new segments, which cannot arise from a broader 

 base than its opening, these are at first very minute, though 

 they rapidly increase in size. The segments are separated 

 by the elongation of the connecting tube, which is converted 

 into two roundish hyaline lobules. These lobules increase 



