HISTOLOGY AND CYTOLOGY 363 



chromatin, and is regarded as forming a reserve from which the 

 chromatin is recuperated as it is used up. The other is termed 

 plastin, and this is harder to differentiate by means of staining, but 

 it behaves differently during division. Within the nucleus is 

 usually one or several distinct rounded bodies that may be of two 

 different kinds. The nucleolus is a rounded mass of plastin and 

 takes no part in division. The karyosome appears to consist of a 

 plastin basis with which is incorporated a greater or less amount of 

 true chromatin, and so it takes part in division. 



Lastly, we have the fluid portion of the nucleus which appears 

 to be somewhat more liquid than that of the general cytoplasm, and 

 is termed the nuclear sap or enchylema. 



Having considered the structure of the cell and the nucleus, 

 in what is termed the resting condition, we can examine the manner 

 in which it divides. It has been noted that reproduction in the 

 Protozoa is brought about by cell division, and that the multi- 

 cellular condition of the Metazoa results from the same process, 

 only the daughter cells as they are produced do not separate, but 

 remain together to form layers. Two distinct kinds of cell division 

 occur, but both of them agree in that it is the nucleus that initiates 

 the activities, and that the cytoplasm follows. The first method is 

 the simplest, and is known as direct division or amitosis, in order to 

 contrast it with the second variety, which is termed indirect division, 

 mitosis or karyokinesis. 



The first indication of direct division is the elongation of the 

 nucleus, which is followed by a lengthening of the cell in the same 

 plane. The nucleus then becomes dumbbell-shaped and finally 

 breaks into two, one part going to each end of the cell. Shortly 

 after this a constriction appears in the middle of the cell, which 

 gradually deepens and finally divides it into two, each of which 

 contains a nucleus. Although a simple form of division, it is a 

 comparatively rare one. It occurs, as we have seen, in the macro- 

 nucleus of Paramcecium, possibly in some forms of Amoeba and in 

 the higher animals in certain pathological conditions, such as 

 cancerous growths. 



Indirect or mitotic division is more complicated, and its details 

 vary slightly in different animals ; but there is a sufficient general 

 resemblance to enable us, for descriptive purposes, to divide the 

 process into four more or less distinct stages, namely, the prophase, 

 the metaphase, the anaphase and the telophase. 



The first indication of the approaching division is given by the 

 centrosome, which, if single, divides into two granules that move 

 apart, or, if double already, they start to separate. When they have 

 moved about a short distance each is surrounded by a series of 



