304 



EVOLUTION, GENETICS, AND EUGENICS 



This separation is followed by a similar constriction and final division 

 of the entire cell-body, which results in the production of two new 

 cells. This form of cell-division is known as simple or direct division. 

 Such a simple division, while found in higher animals, is less frequent 

 and apparently much less significant than another type of division 

 which involves profound changes and rearrangements of the nuclear 

 contents. The latter is termed mitotic or indirect cell-division. Fig. 44 

 illustrates some of the stages which are passed through in indirect 

 cell-division. The centrosome which lies passively at the side of the 

 nucleus in the typical cell (Fig. 44, a) awakens to activity, divides and 

 the two components come to lie at the ends of a fibrous spindle. In 



Centrosome. 



Nucleus. 



Chromatin. — >■ 



True ; 



nucleolus 

 (plasmosome). 



Chromatin 1 



nucleolus. \ 



Linin — f 



network. ^» 



Plastids. 



Vacuole. 

 M etaplasm 

 (passive bodies). 



Fig. 43. — Diagram of a cell, showing various parts. {From Gtiyer.) 



the meantime, the interior of the nucleus is undergoing a transforma- 

 tion. The granules and patches of chromatin begin to flow together 

 along the nuclear network and become more and more crowded 

 until they take on the appearance of one or more long deeply- 

 stained threads wound back and forth in a loose skein in the nucleus 

 (Fig. 44, h) . If we examine this thread closely, in some forms it may 

 be seen to consist of a series of deeply-stained chromatin granules 

 packed closely together, intermingled with the substance of the 

 original nuclear network. 



As the preparations for division go on the coil in the nucleus breaks 

 up into a number of segments which are designated as chromosomes 

 (Fig. 44, c). The nuclear membrane disappears. The chromosomes 



