ii2 COMPARATIVE ANATOMY 



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 

 granules over an achromatic reticulum or network, which often 

 exhibits thickenings or nodes at the points where the linin 

 threads are, as it were, knotted together to form the net- 

 work. These thickenings are not to be confounded with the 

 nucleolus, whose share in the process of mitosis is somewhat 

 obscure. 



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 positions at opposite 

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

 centrosome becomes surrounded by a number of radiating 

 fibrilke 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. 



Meanwhile important changes have been going on in the 

 nucleus. The linin network breaks up and resolves itself 

 into a convoluted thread known as the skein or spireme. The 

 chromatin increases in quantity, possibly at the expense of the 

 nucleolus, stains more intensely than before, and is distri- 

 buted in the form of granules or minute discs along the linin 

 thread. In a short time the spireme breaks up into a number 

 of segments of equal length, which may be straight and rod- 

 shaped, or curved into a horseshoe shape, or may be simply 

 spherical or ovoid masses. In some cases the curved rods 

 may be joined together by their ends to form rings. But 

 their shape is relatively unimportant, the essential thing is 

 that the spireme is resolved into a definite number of equal 

 masses, called chromosomes, and it has been asserted that 

 every species of plant or animal has a fixed number of 

 chromosomes, which number recurs in every division of every 



