12 HAND-BOOK OF PHYSIOLOGY. 



In the frog's ovum (in which the process can be most readily ob- 

 served) after fertilization has taken place, there is first some amoeboid 

 movement, the oscillation gradually increasing until a permanent dimple 

 appears, which gradually extends into a furrow running completely round 

 the spherical ovum, and deepening until the entire yelk-rnass is divided 

 into two hemispheres of protoplasm each containing a nucleus (Fig. 4, b). 

 This process being repeated by the formation of a second furrow at right 

 angles to the first, we have four cells produced (c): this subdivision is 



FIG. 4. Diagram of an ovum (a) undergoing segmentation. In (6) it has divided into two; in (c) 

 into four; in (d) the process has ended in the production of the so-called u mulberry mass." (Frey.) 



carried on till the ovum has been diyided by segmentation into a mass 

 of cells (mulberry-mass) (d) out of which the embryo is developed. 



Segmentation is the first step in the development of most animals, 

 and doubtless takes place in man. 



Multiplication by fission has been observed in the colorless blood-cells 

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

 commence with the nucleolus which divides within the nucleus. The 

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

 dering 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 



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



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 ex- 

 ception 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 num- 

 ber 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 i 'endogenous fission," but there seems to 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 com- 

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



