REJUVENESCENCE IN NATURE. 247 



if in contact with the cuticle. All this may be very 

 exactly traced out in the larger species of Spirogyra ; and 

 I add, in conclusion, that all the subdivisions of the cell- 

 membrane (exclusive of the cuticle) swell up most 

 strongly in the vicinity of the intercellular passages. 



A similar gradual progression of the cell-division will 

 doubtless be found in other Confervas not yet accurately 

 examined in this respect. Observation of the process of 

 division in (Edogonium and Ulolhrix, which genera have 

 a lateral nucleus, would be very interesting. The rela- 

 tionship which exists between the Desmidiaceae and the 

 Zygnemacea3, as also the occurrence of a central nucleus 

 observed in many of the genera of that family, leads to 

 the conjecture that their division corresponds to the pro- 

 cess in Spirogyra. In the Diatomaceae the presence of a 

 nucleus is still doubtful; the age at which the division 

 takes place, however, leads us to expect a gradual pro- 

 gress of the division. Most genera of the two families 

 last named, present us, at the same time, with examples 

 of complete separation of the two cells formed by division, 

 the membrane of the parent-cell either gradually vanishing 

 or being peeled off. (See pp. 180, 181.) 



b. Halving with (apparently /) simultaneous definition 

 of the entire surface of division. Young-cell-division. 

 This is the ordinary mode of formation of the cells in 

 young developing organs of the Phanerogamia,* Vascu- 

 lar Cryptogamia, Mosses,f Characeae, Moridese,j and 

 Fucoidea?.^ The formation of the special mother-cells of 

 the pollen of the Phanerogamia, | and of the spores of 

 the Cryptogamia, when this takes place through repeated 

 halving, also belongs here ; in like manner the formation 



* See, for instance, the course of development of the ovule of Orchis, in 

 Hofmeister, ' Die Enstehung des Embryo der Phanerogamia,' pp. 1 3, t. i. 



f Nageli, ' Zeitschrift,' 1845, p. 138, t. ii, iii. 



J Ibid., 1845, p. 119, t. i, (' Growth of Delesseria Hypoglossum") and 1847, 

 p. 207, t. vi, vii (Polysiphonia.) 



Ibid., 1844, p. 73 et seq. 



|| Nageli, 'Entwicklungsgeschichte des Pollens,' 1842 ; Hofmeister, 'Ueber 

 die Entwicklung des Polleiis,' ('Bot. Zeitung,' 1848, pp. 425, 649.) 



