56 



GENERAL EMBRYOLOGY 



general at right angles to the preceding division (Fig. 24). Thus 

 the planes of succeeding divisions tend to alternate, each per- 

 pendicular to the preceding. Any other relation involves the 

 migration of the centrosome from the position originally 

 occupied by it in the daughter cell; or the spindle may change 

 its position during or after its formation, and this regular 

 relation thus be disturbed. Typically the spindle takes such 

 a position that its long axis lies in the direction of the greatest 

 protoplasmic extent of the cell (0. Hertwig), a position which 

 would result from the tensions between a comparatively 

 elongated body and a fluid medium in which it is suspended 



III 



FIG. 24. Diagram illustrating the relation between the position of the centro- 

 some and the plane of cell division. Symmetrical motion of the daughter centro- 

 somes results in the regular alternation, at right angles, of successive division 

 planes. 



and free to move in any direction. There are many ex- 

 ceptions to these two general rules in special conditions, such 

 as simple columnar epithelia, stratified epithelia, the maturing 

 germ cells, etc.; these indicate perhaps that a more funda- 

 mental cause of the direction of cell division remains to be 

 discovered. 



Some important modifications of this schema of cell division given 

 above will be noted in other connections, but a few special conditions 

 are conveniently mentioned here. The most important and fundamental 

 modifications are doubtless those which occur during the forming and 

 maturing of the germ cells ; these are to be described in detail in Chapter 

 IV, and here we should only note that the behavior of the chromosomes 



