THE CHANGES DURING CELL DIVISION 165 



some cases, an extremely minute body is found outside but 

 close to the nucleus. This body consists of a central granule, 

 and is usually surrounded by rays. The granule is called the 

 " centrosome," and the rays the "aster." 



I. The Changes during Cell Division. 



Take a prepared and stained section of the newt's testis, or of 

 some other actively dividing tissvie, in order to see the structures 

 mentioned below. Giving to the fact that the plane of this 

 section cuts many of the cells at very different angles, the nuclear 

 figures even at the same phase of division will appear very 

 different. Examine and draw under a high power. 



1. Prophase. 



A. The nuclear changes. 



The chromatin substance increases in staining power, loses 

 its net-like arrangement, and gives rise to a definite number 

 of separate intensely staining bodies known as chromosomes 

 (Fig. 39). In many cells the chromatin takes the form of a 

 spirally wound thread (spireme) before it is broken up into 

 chromosomes. 



A striking fact about the chromosomes is that their number 

 is constant in every somatic tissue-cell of a given species of 

 animal or plant, and that this number in all organisms that 

 reproduce sexually is an even number. 



B. The achromatic figure. 



While the chromosomes are arising, a series of changes is 

 taking place at the periphery of the nucleus. The centrosome 

 of the undivided cell splits into two, and the two centrosomes 

 move apart as a spindle of delicate fibres arise between them. 

 Meanwhile the outer wall of the nucleus has become less 

 distinct, and the nucleoplasm is mingling with the cytoplasm. 



Around each centrosome a number of excessively delicate 

 radiating processes are forming the astral rays so that the 

 spindle appears to be giving off radiations at its ends. This 

 figure is the amphiaster or achromatic figure, and occupies 

 the position held formerly by the nucleus. 



