FORMATION OF CELLS. 



15 



present which have been formed by the division of the nucleus of the mother-cell. 

 The next stage is that represented in II, where a lamella of cellulose, which has been 

 formed in the cell-plate between the two nuclei, completely divides the mother-cell. 

 Thickening takes place at the junction of this partition- wall with the wall of the 

 mother-cell, and the protoplasm of the two daughter-cells becomes rounded off. In 

 IF the nucleus of each daughter-cell is seen to have divided into two; sometimes this 

 division does not take place in one of the cells (F). A cell- wall is now formed simul- 

 taneously between each pair of nuclei : a proper cell-wall is subsequently formed round 

 each mass of protoplasm, the thick investing cell-walls undergo absorption, and the 

 pollen-grains are set free.] 



The four cavities into which the mother-cell is divided in the development of spores 

 and pollen-grains were at one time desig- 

 nated the 'Special mother-cells' of the 

 pollen-grains, in which the grains them- 

 selves were formed. The term is however 

 inaccurate, because the four protoplasm- 

 masses are themselves the essential parts 

 of the new cells, becoming subsequently 

 enclosed in cell-walls. If the contents of 

 the cavities are termed special- mother- 

 cells, they are identical with the daughter- 

 cells, /'. e. the pollen-grains ; if, on the other 

 hand, the term is applied to the walls of 

 the cavities, this is not in accordance with 

 the present cell-theory. The term is in 

 fact altogether superfluous. 



c. Cell-formation by Budding and Ab- 

 striction. Among Fungi certain kinds of 

 reproductive cells, conidia,^stylospores, and 

 basidiospores, are produced by small wart- 

 like protuberances on a mother-cell, which 

 then swell out and become rounded. The 

 daughter-cell thus formed retains however 

 at first its connection with the mother- 

 cell at its base, the contents of the two 

 being connected by a channel through this 

 narrow portion. A septum is eventually 

 formed across this channel which splits 

 into two lamellae, and in this manner the 

 separation of the spore from its mother- 

 cell is effected. The production of these spores therefore commences by a budding and 

 IS completed by division ; and the whole process may be termed Abstriction. It 

 occurs in a typical form in the reproduction of the yeast-fungus {Saccharomyces). 

 A wart- like out-growth from which several spores may be abstricted one after another 

 is called a Sterigma ; when the mother- cell bears several sterigmata, as in the Basi- 

 diomycetes, it is termed a Basidium. Intermediate processes between cell-formation 

 by abstriction and ordinary cell-division occur when the protuberance is broad, and 

 the daughter-cell therefore attached to the mother-cell by a broad base and separated 

 from it by a broad septum, as in the branching oi Cladophora ; or, on the other hand, 

 when the terminal portion of a hypha is divided by septa, and the resulting cells 

 become rounded off" and detached, as in the formation of the spores of Cystopus, 

 JEcidium, and other Fungi. [When the budding cell contains a single nucleus this 

 probably divides, and one of the new nuclei goes to the bud ; when the cell contains 

 several nuclei some of these travel into the bud.] 



Fig. 12.— /—F// Successive stages in the formation of the 

 pollen of Funkia ovata (X 550). In VII the wall of the 

 daughter-cell has absorbed water till it has burst ; its proto- 

 plasm is forcing itself through the cleft, and is becoming 

 rounded off. 



