80 LECTUKES ON BIOLOGY 



adult stage, they have nevertheless descended from typical 

 nucleated cells, having lost the nucleus as they grew up : as 

 their task has been fulfilled they are no longer in need of it. 

 After having circulated for a short time through the arteries and 

 supplied the tissues of the body with oxygen from the lungs these 

 red blood-corpuscles decay and are disintegrated in the liver, 

 other new corpuscles taking their places. When they are young 

 we are able to observe quite distinctly the process of reproduction. 

 First the body of the cell extends longitudinally and the nucleus 

 assumes the form of a dumb-bell. The groove in the centre 

 grows deeper and deeper, and soon we observe in the proto- 

 plasm, in the place of one large nucleus, two smaller nuclei. 

 Now the two nuclei commence to move, each to an opposite 

 pole. Here they remain, for their part in the process of 

 division is over. While these alterations went on in the nucleus 

 the protoplasm did not remain idle. We saw that the cell- 

 body commenced to extend longitudinally. Soon it begins to 

 constrict in the centre of its axis, and the originally round 

 blood-corpuscle gradually assumes dumb-bell form. We have now 

 before us two small nucleated cells held together by a thin cord of 

 protoplasm. Finally, this last connecting link is severed and the 

 act of reproduction is over : one blood-corpuscle has become two, 

 which soon replace by growth the loss of substance and then 

 proceed in their turn to division, or develop into red blood- 

 corpuscles. 



But division proceeds only in rare cases in this simple manner, 

 for only cells whose descendants will shortly cease to live are able 

 to transmit to them parts of the nuclear substance in this rough- 

 and-ready manner. That with this primitive method of multipli- 

 cation the distribution of the nuclear substance is very unequal 

 must be clear to everyone. We see, however, that as a general 

 rule the process of division is much more difficult. In place of 

 the crude constriction there is a mechanism which carries out the 

 division with wonderful accuracy, effecting an absolutely just 

 and equal distribution of the chromatin. This process is called 

 indirect nuclear division, or mitosis. Generally mitosis takes 

 place in four different phases which have been described as 

 prophasis, metaphasis, anaphasis, and telephasis. 



In a resting cell (compare fig. 20) all the chromatin is 



