THE VITAL PHENOMENA OF THE CELL 223 



the focus ; that is to say, the corresponding cell divisions only 

 occur after the periclines below the centre of curvature have 

 become sufficiently far apart from one another for it to be neces- 

 sary for new periclines to intercalate themselves between them ; 

 and the same is true of the anticlines. It is easy to see in the 

 diagram (Fig. Ill), that the curvatures of the consti-uction lines 

 are especially sharp around the common focus of all the anticlines 

 and periclines." 



" Hundreds of median longitudinal sections, through the growing- 

 points of roots and shoots, have been made by various observers, 

 before the fundamental principle was at all understood, and all 

 of these correspond with the construction which I have given, and 

 thus prove its accuracy." 



Finally, in order to understand certain variations from normal 

 cell division, a fourth law must be mentioned, which has been 

 formulated by Balfour (VI. 3) in the following words : " The 

 rapidity with which a cell divides is proportional to the concentra- 

 tion of the protoplasm it contains. Cells rich in protoplasm 

 divide more quickly than those which are poor in protoplasm and 

 rich in yolk." This law is explained by the fact that, in the 

 process of division, it is the protoplasm alone Avhich is active, the 

 yolk substance stored up in it being passive, and, so to speak, 

 carried along by the active protoplasm. The greater the amount 

 of yolk present, the more work is there for the protoplasm in 

 division ; indeed, in many cases there may be more to do than the 

 protoplasm can accomplish. This occurs frequently in eggs, in 

 which polar differentiation has occurred, the greater part of the 

 protoplasm being concentrated at the animal pole. Then division 

 is confined to this portion of the cell, the vegetative part being no 

 longer broken up into cells. Thus an incomplete or partial division 

 has resulted instead of a complete one. Both extremes are united 

 in nature by intermediate forms. 



2. Review of the Various Modes of Cell Division. 



The following classification, upon which I have based my detailed 

 accounts, may be made of the various methods of cell division. 



I. COMPLETE OR HOLOBLASTIC SEGMENTATION. 



a. EQUAL. 



b. UNEQUAL. 



c. CELL-BUDDINO. 



