REJUVENESCENCE IN NATURE. . 283 



reproductive cells, the transition-cells to. a new vegetative 

 cycle.* In the preceding summary of the conditions of 

 cell-formation, I have purposely avoided mixing up those 

 caused by conjugation ; as the opposite of all kinds of 

 cell-formation combined with division, they deserve a 

 separate examination, through which, from the insuffi- 

 ciency of most of the descriptions of the cases belonging 

 here, I purpose, at the same time, an attempt at placing 

 the subject in its proper light (Orientirungs-versuch), to 

 guide in future investigations. In the following collo- 

 cation we restrict the idea of conjugation to those cases 

 in which: 1, two previously really separate cells unite ;f 

 2, the combination takes place by actual anastomosis, not 

 by mere application together of the cell- walls ;J 3, an 

 immediate intermixture of formative cell-contents occurs. 

 Thus, in true conjugation, two previously separate cells are 

 united in such a manner that in the combined state they 

 can only be regarded as one cell, the two halves of which 

 sometimes remain distinguishable, connected by a narrow 

 isthmus or by a canal (trabecula), but sometimes coalesce 

 so completely that the earlier boundaries entirely disap- 

 pear. In such union, either all parts of the combining 

 cells (membrane and contents) enter into conjugation, or 

 the cell-membrane, and then either the whole or only the 

 outer, remains out of the combination, bursting so as to 

 let the inner membrane or merely the contents (with the 

 primordial utricle) conjugate. In this case it may happen 

 that the contents divide within the cell-membrane shortly 

 before the conjugation, so that two cells are formed 



* See above, p. 132. (Diatomacese) ; pp. 133 135, (Desmidiaceae, 

 Zynemaceae.) 



f This excludes the phenomena in the formation of the spores otMelosira, 

 (Edogonium, and Bulbocheete, subsequently to be examined more closely, which 

 have been mixed up with conjugation. 



J This excludes the mere application of other cells upon the mother-cell 

 of the spores, such as occurs in Coleochtete and Saprolegnia, as also the 

 union of the pollen-tube with the embryo-sac. 



This excludes the anastomosis of the colourless cells of the leaves of 

 'Sphagnum, Dicranum glaucum, and certain other white-leaved Mosses, as 

 also the union of long tubular spirai-flbre-cclls to form vessels. 



