CHAPTER I. THE CELL. 



115 



26.] 



The following two modes of cell-formation may be distin- 

 guished : 



1. Cell- formation without division of the cytoplasm. 



2. Cell-formation with division of the cytoplasm. 



1. Cell-formation without division. This process does not lead to 

 the multiplication of cells, but merely to the development of a new 

 cell. Two cases are distinguishable : (a) that in which the new 

 cell is formed from a single cell, that is, direct cell -formation ; (b) 

 that in which the new cell is formed by the fusion of the cyto- 

 plasm of two cells, that is, cell-formation by conjugation. 



(a) In direct cell-formation without division, the cytoplasm 

 simply undergoes reorganisation and enters upon a new individu- 

 ality. It is confined to the development 

 of reproductive cells ; such as zoogonidia 

 and spores, the spermatozoids of the 

 higher Cryptogams, and oospheres in 

 many cases. This particular case of cell- 

 formation is sometimes termed rejuven- 

 escence. 



A good illustration of this is afforded by the 

 development of the zoogonidium of (Edogonium. 

 The cytoplasmic contents of an ordinary cell of 

 the filament become more abundant ; the kino- 

 plasm (see p. 95), with the nucleus and centro- 

 sphere, travels to one side of the cytoplasm, ap- 

 pearing at the surface as a hyaline, colourless 

 area ; the cytoplasm begins to contract away 

 from the wall (Fig. 74 J, lower cell), and secretes 

 a delicate cell-wall of its own ; the cilia are de- 

 veloped in a circlet from the margin of the kino- 

 plasrnic area. The cell-wall is ruptured (as at Z), 

 and the zoogonidium, Ijing loosely in its own 

 proper cell-wall, as in a vesicle, is extruded into 

 the water. It now begins to move inside the 

 vesicle by the action of its cilia ; the vesicle becomes disorganised, and the 

 zoogonidium is set free. It swims by means of its cilia (Fig. 74 B) with the 

 colourless end in front ; so that what was the transverse axis of the mother-cell 

 has now become the longitudinal axis of the zoogonidium, an indication of the 

 complete reorganisation which has taken place. After a short period of move- 

 ment, the zoogonidium comes to rest, attaches itself to some solid body by its 

 hyaline end, withdraws its cilia, and secretes a cell- wall (Fig. 74 C). 



The plants in which there are highly differentiated spermatozoids, each de- 

 veloped singly in a mother-cell, are the Characeae (Algae), the Bryophyta, and 

 the Pteridophyta. In all these plants the development of the spermatozoid pro- 



PIG. 74. Rejuvenescence as 

 exhibited in the formation of 

 the zoogoiiidium of (Edogonium. 

 A portion of a filament ; in the 

 lower cell the protoplasm is be- 

 ginning to contract, in the upper 

 the young zoogonidium is escap- 

 ing (Z). B a zoogonidium. C 

 the beginning of germination, 

 (x 350). 



