REJUVENESCENCE IN NATURE. 161 



unsupported by any observations. The birth of the 

 germ-cells of Ulothrix zonata takes place in the following 

 way: The mother-cell opens laterally by the tearing of 

 the cell- membrane ; the innermost delicate layer of the 

 membrane of the mother-cell does not share in this rent, 

 but, swelling and expanding by absorbing water, and 

 loosened by this expansion from the outer membranes of 

 the mother-cell, it protrudes in the form of a sac. Only 

 rarely does this sac tear during its protrusion, and let out 

 the germ-cells singly: in most cases it by degrees 

 emerges entire, and, as a globular vesicle, still holds 

 the germ -cells, already actively moving, imprisoned for 

 some time in its interior. Finally the sac bursts, and 

 the germ-cells separate rapidly in all directions. Even 

 after the emptying it may be still detected as a delicate 

 water-clear vesicle, without the slightest trace of 

 secondary mother-cell membranes in the interior. Some- 

 times miscarriages occur, demonstrating the absence of 

 the latter still more absolutely. It happens, namely, 

 sometimes, that the germ-cells do not become detached 

 from the mother-cell vesicle, but remain coherent with it. 

 Such vesicles are born like the others ; the included germ- 

 cells become separated from one another by the expansion 

 of the vesicle, without being able to leave its walls. In 

 these cases, again, no trace of interposed membranes can 

 be seen. I have observed a very similar process, without, 

 however, a tearing of the vesicle, in which indeed the 

 active germ-cells combine into a regularly-arranged 

 colony, in Pediastrum* 



In (Edogonium^ only a single germ-cell is formed in 



envelope is formed, often alternating with more delicate membranous 

 lamellae, in wbich, therefore, the omission of the formation of these envelopes 

 is very striking. See, as an example, the figure of Glaeocystis vesiculosa in 

 NageU's ' Einzelliger AJgen,' t. iv, F. The multiplication takes place through 

 simple or double halving, i. e. vegetative development and formation com- 

 mences sometimes directly after the first division into two, sometimes after 

 the second ; the two transitory cells first formed producing no membrane. 

 The first case is represented in fig. n, the last in figs, o and/ of the plate 

 cited. 



* Vide plates iii and iv, and the detailed description in the Appendix. 



f (See Thuret, 'Ann. des Sc. iiat.,' 3 ser., xiv, 17, pi. xix. A. H.) 



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