THE VITAL PROPERTIES OF THE CELL 281 



the process of dissolving is quite completed, the 

 two halves of the zygote gradually assume the 

 shape of a Closterium cell (Fig. 152). 



What is the significance of this second 

 division, which occurs immediately after the 

 first, without any intermediate resting stage ? 

 It appears to me that by its means the same 

 result is obtained, although in a different 

 manner, as is produced by the division, with p ie . 152. TWO 

 reduction, which occurs during the maturing Ciosteria, which have 



\ developed from a 



of egg and sperm-cells. In both cases by Zy go 8 pore, before 

 means of the double division the nuclear sub- they have escaped 



, , , ii.cr!j.ij. j. j from their enclosing 



stance is reduced to one half of that contained mem brane. 

 by a normal nucleus, and thus an increase of 

 nuclear substance is avoided when, in consequence of fertilisation, 

 two nuclei coalesce. Similarly in Desmidiacede a reduction of 

 nuclear substance occurs after fertilisation, and thus the double 

 amount of nuclear substance, produced by the conjugation of 

 two complete, fully developed nuclei, is reduced to a normal 

 quantity. The germinal nucleus, instead of dividing into two 

 daughter-nuclei, splits up in consequence of the two divisions, 

 which follow immediately upon one another, into four grand- 

 daughter-nuclei. The protoplasmic body, however, is halved, 

 each portion containing only one functional nucleus ; the other 

 two, being useless, disappear. 



This supposition might be proved to be correct, if the nuclear 

 segments were accurately counted at the various stages. One 

 circumstance, which may be mentioned in its support, has fre- 

 quently been observed by Klebahn, namely that in Cosmarium 

 the four granddaughter- nuclei, which are derived from the ger- 

 minal nucleus, are distributed unequally between the halves of 

 the zygote, the one half containing one single active nucleus, and 

 the other containing three, two of which degenerate. It does not 

 matter whether the two degenerating nuclei fall to the share 

 of one or both cells during division, since they behave like yolk 

 contents. 



In Desmidiacese we have observed conjugation as it occurs in 

 isolated living cells ; the Zygnemacese teach us its method of pro- 

 cedure in a colony of cells, where several individuals have joined 

 together in rows to form long threads. 



When, in the thick felt-like masses with which the Algae cover 



