26.] 



CHAPTER I. THE CELL. 



125 



t:-r\ n 



pleted, a cell-plate is formed in the kinoplasmic spindle, and in 

 and from this a septum of cellulose is formed (Fig. 82, 7-11). 

 If the spindle is not wide enough to reach quite across the cell, 

 it is extended laterally by the formation of new connecting-threads, 

 until the septum is attached to both lateral walls. Thus the 

 walls of the new cells are, for the most part, those of the parent 

 cell, the septum being the only new formation. It is a universal 

 law that the plane in which the septum is formed is perpendicular 

 to that of the wall at any point of contact. 



Variations of this mode of cell-formation occur in isolated cells, and in cells 

 forming part of a free surface. In some cases (e.g. branching of Moss-proto- 

 nema) the mother-cell throws out 



a lateral protuberance, and a wall r& 



is formed at the junction of the 

 two. In other cases, as in the 

 development of the spores of 

 many Fungi, and in the multipli- 

 cation of Yeast-cells, the lateral 

 protuberance expands into a 

 rounded body, the neck of com- 

 munication with the mother-cell 

 remaining narrow. Eventually 

 a cell-wtll is foimed across the 

 narrow neck, and the cells sepa- 

 rate from each other. This mode 

 of cell-formation is known as 

 gemni'ition or abstraction. 



Closely connected with 

 the division into two, which 

 is characteristic of cell- 

 formation in the vegetative 

 parts of plants, is the divi- 

 sion of the mother-cell into 

 four, which is characteristic 

 of the development of the 

 spores in the higher plants 

 (spores of Bryophyta, Pteridophyta, and pollen-grains of Phanero- 

 gams ; see pp. 117, 123) ; in these cases the division of the mother- 

 cell may take place in either of the two following ways : 



1. Each nuclear division is followed by cell-division with the 

 formation of a septum (Fig. 84 A ; also Fig. 78) ; hence the 

 process of the development of the four special mother-cells is one 

 of successive cell- division, the mother-cell being first of all divided 



FIG. 83. Early stage of cell-division in a young 

 endosperm- ee II of Fritillaria imperialis (diagram, 

 matic ; after Strasburger : x 1000) . n n Two recently 

 formed nuclei with connecting threads between 

 them; the threads show median thickenings 

 which will constitute the cell-plate cp ; p cytoplasm. 



