REJUVENESCENCE IN NATURE. 299 



Distant as the physiological importance of these pheno- 

 mena of application just described stands, from the appli- 

 cation of the pollen-tube on the embryo-sac, and through 

 the medium of this, on the germinal vesicle, connected 

 with the fertilisation of the Phaneroganria, the resem- 

 blance between the processes of the two phenomena is 

 unmistakeable. 



Some of the Diatomacea? afford another case, to be 

 placed here as it were in an appendix, which perhaps 

 might be regarded as an instance of arrested conjugation. 

 While I have seen in many genera of this family an 

 actual conjugation of completely separate cells (p. 286), 

 in the group of the Melosirete the large reproductive-cell 

 which commences the new series of generations (the 

 sporangium,* as it is termed), is formed out of the con- 

 tents of a single grown-out cell, the shell or membrane of 

 which, dividing in the middle, separates to allow space 

 for the expansion of the newly-formed, more vigorous 

 primary-cell. Thwaites,f who describes this process in 

 several species, thinks that it may be brought into 

 agreement with the conjugation occurring in other 

 groups of the same family, by assuming the division of 

 the contents of the mother-cell into two portions pre- 

 viously to the formation of the new cell, these portions, 

 instead of developing into separate cells, as in the vege- 

 tative increase by division, combining together again im- 

 mediately in order to product the reproductive-cell. 

 Although this hypothesis is not yet substantiated by 

 direct observation, it deserves attention, since it opens 

 a common point of view for the phenomena of reproduc- 



* See p. 141, note. 



f See Thwaites, ' Further Obs. on Diatomacese,' ' Annals of Nat. Hist.,' 

 see. ser., vol. i, p. 161, t. xi, xii, (1848.) The reproductive cell is formed either 

 between the halves of an isolated cell, separating from them in the sub- 

 sequent development, (Cyclotella, t. xi, f. D,) or between the halves of a cell 

 which is a link of a filament, sometimes parallel and becoming developed in 

 connection with the old filament, (Melosira, t. xi, f. A, c, and Orthrosira, 

 t. xii, f. E,) sometimes pushing aside the halves of the mother-cell by 

 secretion of gelatinous matter, and developing at right angles to the old 

 filament, (Aulocosira, t. xi, f. B.) 



