140 JOHN PRIESTLEY. 



thus causing a greater divergence of the nuclear fibres con- 

 nected with it, and then splits into two lamina? — the skins 

 [Hautschichten) of the coming cells, between Avhich cellulose 

 is developed. Hence this plate is called the cell-plate 

 [Zellplatte). Division is completed beyond the circumference 

 of the cell-plate by a, like differentiation of the protoplasm 

 existing there. 



The above-mentioned phenomena of division, which 

 constitute a favorable illustration for comparison with 

 division in animal cells, were only studied in alcoholic 

 preparation in the case of Picea. No doubt of their continuity- 

 can, however, exist, since in the large- celled freshwater 

 alga, Spirogyra orthospira, Nseg., with its suspended nucleus 

 and cavernous or tunnelled cell-masses, the similar phenomena 

 may be followed with great ease in the living plant under 

 the microscope. 



In the animal kingdom, Avhere division seems to be the 

 only well- authenticated mode of increase of cells, Strasburger 

 examined the ova of Phallusia mamillata. 



On the disappearance of the original nucleus a peripheral 

 lenticular clear body developes. Of this the central part 

 dips into the substance of the ovum and divides off as the 

 new nucleus, which soon presents a vacuole containing nu- 

 cleoli. The nucleus of the first cleavage mass, therefore, is 

 not here formed by the union of two or more pronuclei 

 (Pl.XII, figs. 13andl4). 



"With the appearance of the clear disc the cell-protoplasm 

 had become radiate, the rays proceeding from the disc ; now, 

 the radiation centres about the nucleus. 



The latter soon becomes homogeneous and elongates ; and 

 finally exhibits a striation of a few converging lines with equa- 

 torial enlargements (PI. XII, fig. 15). Strasburger did not 

 himself succeed in observing an actual splitting of a nuclear 

 disc ; but Biitschli, in the cases of Cuculla?ms elegans and 

 Blatta germanica, has figured this stage unmistakeably (PI. 

 XII, figs. 17 and 18)^. The new nuclei are formed much as 

 in plants. 



The stellation of the cell-mass is now twofold, proceeding 

 from two centres ; and certain of the rays from both centres 

 intercross about the middle of the ovum. The protoplasm 

 gradually clears up in the equatorial plane, whereupon division 

 commences as an inversion of the edge of the ovum at the 

 periphery of that plane, and is often completed as a sudden act. 



The rejuvenescence of cells takes up but a few pages of 



^ Stasbuger had Biitschli's permission to insert a few figures from an un- 

 published work by the latter. 



