232 



THE CELL 



III. So-called Free Cell-Formation. 



The peculiarity of this form of multiplication consists in this, 

 that the nucleus in a cell subdivides several times consecutively, 

 Avhilst the protoplasmic body remains undivided for a considerable 

 time without showing the least inclination towards even a partial 

 cleavage. After bipartition has been repeated several times, the 

 number of nuclei in a single protoplasmic body may amount to 

 several hundreds. These arrange themselves at regular distances 

 from one another. Finally a period arrives when the many-nucle- 

 ated mother-cell becomes either suddenly or gradually split up 

 into as many daughter-cells as there are nuclei in it. 



Free cell-formation occurs chiefly, in both plants and animals, 

 during the development of the sexual products. In order to 

 demonstrate it, I will select three examples: the superficial 

 segmentation of the centrolecithal eggs of Arthropoda, the for- 

 mation of the endosperm in the embryo-sac within the ovule of 

 Phanerogamia, and the formation of spores in the sporangia of 

 Saprolegnia. 



The yolk mass is generally collected in the centre of the egg 

 in Arthropoda, being surrounded by a thin peripheral layer of 

 protoplasm. Hence the eggs are called centrolecithal, i.e. eggs 

 with yolk in the centre, in distinction to telolecithal eggs, in which 

 the yolk is situated at the poles (Balfour VI. 3). The cleavage 

 nucleus, surrounded by a protoplasmic envelope, is generally in 

 the centre of the nutritive yolk ; here it divides into two daughter- 

 nuclei, whilst the division of the egg itself does not immediately 



B 



C 



Fig. 122. Superficial cleavage of the egg of an insect (Pieris crataegi) (after Bobretsky ; 

 from R. Hertwig, Fig. 100) : A division of the cleavage nucleus ; B the nuclei raise 

 themselves and commence to form a germinal layer (blastoderm) ; C formation of blas- 

 toderm. 



