138 JOHN PRIESTLEY. 



spective destinations the nuclei, which are about 15-18 fx in 

 diameter, acquire nucleoli ; but as yet they remain quite naked. 



Division into two may now be said to be complete. What 

 remains of the bistellate figure gradually disappears, the 

 nuclei alone remaining ; and activity is succeeded by repose. 



The following stages of cleavage are but a repetition of 

 the above- described steps. Division into four, into eight, 

 and into sixteen masses can be traced in detail with com- 

 parative ease ; but beyond this the size of the objects renders 

 observation difficult. 



The theory by which Auerbach explains these appearances 

 is one of a distribution and recollection of nuclear substance. 

 After the disappearance of the original germinal vesicle and 

 the occurrence of fertilisation, two nuclei appear, formed, 

 as, in Auerbach's view, all nuclei are formed, viz. by 

 a vacuolation of the cell mass. These slowly move 

 together, touch, fuse into one, and pass into an enucleolar 

 state. The resulting body, whose method of formation did 

 not fail to suggest to Auerbach the resemblance to conjuga- 

 tion, elongates and becomes smaller, expelling its contents 

 from both ends into the surrounding granular protoplasm ; 

 and the disturbance in the arrangement of the granules gives 

 rise to the appearance of two stars. This act is what 

 Auerbach describes as karyolytic {tcapvov =■ nucleus.) 



The formation of the new nuclei is but a reversal of the 

 above operation. The scattered nuclear substance in each 

 half of the ovum aggregates about a point situated in what 

 will shortly become its proper segment. It is, in reality, a 

 return of the nuclear mass to its former state of aggregation 

 after a temporary sojourn in the interstices of the cell; and 

 Auerbach has therefore designated the process of growth of 

 the young nuclei palingenetic. 



Another work of importance bearing out the life-history 

 of nuclei is one by Prof. Strasburger, of Jena/ published 

 in 1875. 



In it three methods of the increase of cells in the animal 

 and vegetable kingdoms are investigated, viz. free cell- 

 formation, cell-division, and the renewal or rejuvenescence 

 of cells [Vollzellbildung) . 



Of the first method several observations are recorded and 

 figured, of w^hich that of the embryo-sac of Phaseolus multi- 

 florus is typical. There, in the formation of endosperm- 

 cells, the nuclei first appear as punctiform thickenings, 

 surrounded by a clear zone with tolerably defined borders. 

 1 ' Ueber Zellbildung und Zelltheilung.' Jena. 



