78 



PRINCIPLES OF ANIMAL BIOLOGY 



of the chromosomes has its variations. While, as stated above, it 

 typically occurs while the chromosomes are in the equatorial plate, there 

 are many instances in which the chromosomes divide 

 long before they assume an equatorial position. In 

 some species this splitting occurs as early as the spireme 

 stage. 



The chromosomes vary greatly in form. Frequently 

 they are V-shaped. Short or long rods and spherical 

 forms are also common, while some are hook-shaped. 

 In the maturation of eggs and sperms chromosomes 

 usually unite in pairs, and then the pair may assume 

 ringlike and various other forms. 



Division of Cytoplasm. — In the method of dividing 

 the cytoplasm into two parts at the time of mitosis 

 there is a difference between plant and animal cells. In 

 most animal cells there occurs a constriction of the cell 

 body in the equatorial region. This process of constric- 

 tion continues until the cytoplasm is divided into two 

 parts. In plant cells division of the cytoplasm is ac- 

 complished by the growth of a cell plate at the median 

 points of the spindle fibers. The position of the cell 

 plate is first marked by thickenings on the spindle fibers 

 (Fig. 50). These thicken- 

 ings increase in size and 

 number during the telophases and event- 

 ually a new cell wall is laid down in the 

 plane which they occupied. 



Significance of Mitosis. — The signifi- 

 cance of the mitotic method of cell division 

 may be appreciated when one contemplates 

 the fact that the process results in an ap- 

 proximately equal quantitative and quali- 

 tative division of the material of each 

 chromosome. Th e equal qualitative div i- 

 s ion of the chromosom es is es pecially of 

 g reat importance . It is the present dav 

 conception, supported by a considerable 

 body of evidence derived from breeding 

 experiments and observation, that many 



Fig. 49.— In- 

 tranuclear mitosis 

 (anaphase) in the 

 rhizopod proto- 

 zoon Euglypha. 

 {From Wilson's 

 The Cell, after 

 Schewiakoff.) 





XiC^^iM^ 



Fig. 50. — Formation of the cell 

 plate in a dividing cell of the root 

 tip of the hyacinth. The thick- 

 enings on the fibers of the spindle 



of the substances (of whatever sort they %^ the beginning of the process. 



. ... . (t' rom Dahlgren and Kepner s Pnn- 



may be) which determme the activities of dpics of Animal Histology.) 

 the cell and also those which determine 



hereditary qualities (t he genes, Chapter XI), are distributed along the 

 length of the chromosomes. Tt is also believed that the individual 



