134:8 PHYSIOLOGY 



and spermatozoon cannot be regarded as corresponding to complete 

 cells. Before their union or conjugation both male and female germ- 

 cells undergo certain important changes which differentiate them 

 from the ordinary somatic cells of the individual. The essential 

 differences between a germ-cell and a somatic cell can be best seen by a 

 study of the nuclear changes which precede their formation. In divi- 

 sion the nuclei of all somatic cells, whether of plants or animals, undergo 

 a series of changes which, in their broad outlines, are identical through- 

 out both animal and vegetable kingdoms (Fig. 553). The nucleus of 

 the resting cell in its vegetative condition is generally separated from 

 the cytoplasm by a nuclear membrane, and contains irregular masses 

 of a material staining deeply with basic dyes, and known as chromatin. 

 In the cytoplasm of most animal cells may be seen a small particle 

 known as the centrosome. When division is about to take place the 

 clumps of chromatin arrange themselves into a filament which forms 

 a continuous skein, the * spireme stage.' This then breaks up into 

 a number of segments, often V-shaped, the chromatin filaments or 

 chromosomes. Each of the filaments, in large nuclei, may often be 

 seen to be composed of rows of granules. While this change has been 

 occurring the nuclear membrane in most cases disappears, and the 

 centrosome outside the nucleus divides into two parts, which travel 

 to opposite ends of the nucleus. Round each centrosome the cyto- 

 plasm is modified and presents a radiate appearance, the aster, while 

 joining the two centrosomes is a spindle of fine fibres, the achromatic 

 spindle. The V-shaped segments of chromatin arrange themselves 

 in a circle at the equator of the spindle midway between the two 

 centrosomes. Each of the loops then splits longitudinally, and each 

 half travels towards one or other of the centrosomes, thus forming 

 two daughter nuclei. The half -loops then join to form a skein, and 

 may return to the form of a resting nucleus. These different phases 

 in division are presented by all somatic cells, and have received the 

 following names : 



(1) Prophase (the formation of the spireme and of the achromatic 

 spindle, and the breaking up of the spireme into chromatin loops or 

 chromosomes). 



(2) Metaphase (the splitting of the chromosomes). 



(3) Anaphase (the travelling of each half-chromosome to the 

 extremity of tile spindle). 



(4) Telophase (the retrogressive changes leading to the conversion 

 of the chromatin filaments into an ordinary resting nucleus, which are 

 accompanied or preceded by a division of the cytoplasm across the 

 equatorial part of the spindle). 



When the spireme has broken up into separate chromatin loops, it is 

 possible to count them, and it is found that the number present in 



