8 



HANDBOOK OF PHYSIOLOGY. 



subdivision is carried on till the ovum has been divided by segmentation 

 into a mass of cells (mulberry-mass) (d) out of which the embryo is de- 

 veloped. Segmentation is the first step in the development of all the 

 higher animals, including man. 



Multiplication by fission has been observed in the colorless blood- 

 cells of many animals. In some cases (Fig. 5), the process has been 



FIG. 5. Blood-corpuscle from a young deer embryo, multiplying by fission. (Frey.) 



seen to commence with the nucleolus which divides within the nucleus. 

 The nucleus then elongates, and soon a well-marked constriction occurs, 

 rendering it hour-glass shaped, till finally it is separated into two parts, 

 which gradually recede from each other; the same process is repeated in 

 the cell-substance, and at length we have two cells produced which by 

 rapid growth soon attain the size of the parent cell (direct division). 

 In some cases there is a primary fission into three instead of the usual 

 two cells. 



In cartilage (Fig. 6), a process essentially similar occurs, with the 



FIG. 6. -Diagram of a cartilage cell undergoing fission within its capsule. -The process of divi- 

 rsion is represented as commencing in the nucleolus, extending to the nucleus, and at length involv- 

 ing the body of the cell. (Frey.) 



exception that (as in the ovum) the cells produced by fission remain in 

 the original capsule, and in their turn undergo division, so that a large 

 number of cells are sometimes observed within a common envelope. 

 This process of fission within a capsule has been by some described as a 

 separate method, under the title " endogenous fission," but there seems 

 Ho be no sufficient reason for drawing such a distinction. 



It is important to observe that fission is often accomplished with 

 great rapidity, the whole process occupying but a few minutes, hence 

 the comparative rarity with which cells are seen in the act of dividing. 



Indirect cell division. In certain and numerous cases, the division 

 of cells does not take place by the simple constriction of their nuclei and 



