ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



this process is of the complicated kind known as mitosis or karyokinesis, 

 characterized above all by the chromatin becoming condensed in the 

 region of the equator of the nucleus into a definite number of rounded or 

 rod-shaped pieces called chromosomes, each of which divides into two 

 halves, these being distributed to the two daughter nuclei. The details 

 of the process are obscure in Amoeba and its allies and therefore 

 their description will be held over until we are dealing with a group 

 of animals in which they are more distinct (Chap. V.). 



The process of mitosis having been completed (Fig. 4, A-D) the 



Fig. 4. 



Fission of Amoeba (nuclear detail after observations by L. A. Carter). A, Amoeba before the onset 

 of fission ; B, most of the chromatin has become concentrated in chromosomes about the equator 

 of the nucleus ; C, each chromosome has divided into two, the two sets of daughter chromosomes 

 are moving apart and the boundary of the nucleus is becoming indented between them ; D, the 

 nucleus has become completely divided into two daughter nuclei ; E, the two nuclei have moved 

 apart and the cytoplasmic body of the Amoeba is undergoing constriction ; F, the process has been 

 completed. 



Amoeba takes on an elongated form and its cytoplasm becomes con- 

 stricted across between the two daughter nuclei (Fg. 4, E). As the 

 constriction deepens the isthmus connecting the two masses of cytoplasm 

 becomes narrower and narrower until finally it snaps across, leaving in 

 place of the original Amoeba two Amoebae of half its size which gradually 

 move apart and lead their own lives (Fig. 4, F). 



In this process of fission which serves in Amoeba as a corrective to 

 the process of growth we see a good example of the simplest of all types 

 of reproduction, in which increase in the number of individuals is brought 



