190 INTRODUCTION TO CYTOLOGY 



return to the fluid state, continue to cleave along the normal plane 

 through the equator of the cell as if nothing unusual had happened. All 

 of these observations indicate a close dependence of cytokinesis upon the 

 temporary differentiation of semi-solid masses in the egg cytoplasm, and 

 throw much light upon the question of the true nature of the achromatic 

 figure. 



THE CELL WALL 

 Probably the most striking difference which meets the eye in a com- 

 parison of animal and plant tissues lies in the relative degree of dis- 

 tinctness with which the limits of the individual cells may be made out. 

 Animal cells as a rule are separated only by very thin limiting membranes 

 which in many tissues are so delicate as to be scarcely discernible, 

 whereas the cells of plants usually possess conspicuous firm walls, which 

 in the case of woody plants become greatly thickened and afford 

 mechanical support to large bodies. 



The Primary Wall Layer. — Since the time when mitotic cell-division 

 was first carefully studied with the aid of modern methods it has been 

 known that in the cell wall of plants the primary layer, or middle lamella 

 (the "intercellular substance" and " cement" of early writers), is formed 

 in most cases in close connection with the spindle fibers at the close of 

 mitosis. 1 The exact manner of its origin, however, has proved to be a 

 very difficult point to determine, and has formed the subject of a long 

 continued controversy. (See papers of Timberlake and Allen, 1900 

 and 1901.) During the telophases of mitosis the spindle fibers con- 

 necting the two daughter nuclei develop thickenings (Fig. 58, D), enlarge 

 until they come in contact with one another and fuse to form a cell plate, 

 or partition, between the daughter cells. For some time it was thought 

 (Strasburger 1875, 1882, 1884) that the cell plate so formed became at 

 once the middle lamella, upon which secondary and frequently tertiary 

 layers were subsequently deposited by the protoplasts on either side. 

 Strasburger here found support for his theory that the cell wall is essen- 

 tially a transformed layer of the protoplast, in opposition to Nageli and 

 von Mohl, who regarded it as primarily a secretion product. As a 

 result of further researches, however, he later (1898) abandoned this 

 view and adopted an interpretation that had been suggested by Treub 

 (1878), namely, that the cell plate formed by the consolidation of the 

 swellings (" microsomes") on the spindle fibers very soon splits to form 

 the plasma membranes of the two daughter cells, and that there is then 

 secreted between these membranes by the protoplasts a substance 

 which becomes the primary layer, or middle lamella. The correctness 

 of this view was confirmed by the careful researches of Timberlake 

 (1900) and Allen (1901). Timberlake pointed out that in the micro- 



1 Discussion is here limited to the walls of higher plant tissues. The ectoplast of 

 naked cells has been dealt with in Chapter III. 



