THE VITAL PHENOMENA OF THE CELL 



233 



follow. These daughter -nuclei 

 (Fig. 122 A) then divide into four, 

 these four into eight, the eight 

 into sixteen, and so on, whilst the 

 egg as a whole remains unseg- 

 mented. Later on the nuclei sepa- 

 rate from one another, and for the 

 most part move gradually to the 

 surface (Fig. 122 J5), penetrating 

 into the protoplasmic envelope, 

 where they arrange themselves at 

 equal distances from one another. 

 Not until this has occurred does 

 the egg commence to segment, the 

 peripheral layer splitting up into 

 as many cells as there are nuclei in 

 it, whilst the central yolk remains 

 intact, or is only split up at a 

 much later period. This latter 

 occurs when in the eggs of insects, 

 as in telolecithal eggs, the yolk 

 contains yolk nuclei, or merocytes 

 (Fig. 122 G). 



The wall of the embryo-sac in 

 Phanerogamia is coated with a 

 protoplasmic lining, which at a 

 certain stage of development con- 

 tains several hundred regularly 

 arranged nuclei ; these were for- 

 merly considered to develop like 

 crystals in a mother-liquor ; but 

 we know now, that they are pro- 

 duced by the repeated bipartition 

 of a mother nucleus, as in the eggs 

 of Arthropoda (Fig. 123). The divi- 

 sions occur almost simultaneously 

 in any one region of the embryo- 

 sac. If the preparation is suc- 

 cessful, nuclei in numerous stages 

 of division may be observed at one 

 time in a small space (Fig. 123). 



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