112 COMPARATIVE ANATOMY 



bell shaped. The strand connecting its swollen ends becomes 

 thinner, and finally breaks, so that two new nuclei are formed. 

 The cell-body becomes constricted in its middle and eventu- 

 ally is separated into two halves, each containing a portion of 

 the divided nucleus. Such a mode of nuclear division is 

 known as simple or amitotic. It occurs in some Protozoa, in 

 some leucocytes, and characteristically in certain cells forming 

 morbid growths. But in the great m'ajority of cases the 

 process is of a far more complex kind, and it differs somewhat 

 in tissue-cells and in germ-cells. We will take the former case 

 first, as being the simpler. 



A resting nucleus has the characters depicted in figure 23, A. 

 Its nuclear membrane is evident ; and within the membrane 

 the deeply-staining chromatin is distributed in the form of a 

 reticulum or network, which often exhibits thickenings or 

 nodes at the points where the chromatin threads are, as it 

 were, knotted together to form the network. These thicken- 

 ings are not to be confounded with the nucleolus, which does 

 not seem to take an active share in the process of mitosis. 



Outside of the nuclear membrane, lying close to it in the 

 cytoplasm, is a very minute but very important body, known 

 as the centrosome. It is often surrounded by a little 

 specialised mass of cytoplasm, known as the centrosphere. 

 The division of the cell appears to be heralded by changes 

 in the centrosome and the cytoplasm immediately surrounding 

 it. The centrosome divides into two parts, which travel round 

 the periphery of the nucleus to take up provisions at opposite 

 poles. As it divides and travels round, each moiety of the 

 centrosome becomes surrounded by a number of radiating 

 fibrillae stretching into the adjoining protoplasm, and so form 

 ing a characteristic star-shaped figure to which the name of 

 the astral figure has been given, the fibrillae being known as 

 the astral rays. There is some doubt as to whether the astral 

 rays are formed entirely from the centrosphere or from the 

 ordinary cytoplasm. , 



Meanwhile important changes have been going on in the 

 chromatic reticulum of the nucleus. The network breaks up 

 and resolves itself into a convoluted thread composed entirely 

 of chromatin, which now stains even more intensely than 

 before. The convoluted thread at this stage is known as the 

 skein or spireme. In a short time the spireme breaks up into 



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