ESSENTIAL FEATURES OF THE SEXUAL PROCESS 1257 



THE FORMATION OF GERM CELLS 



In multicellular organisms the cells which, conjugate to form a new cell, 

 capable of developing into an individual, are of two kinds. One, which 

 has generally a certain amount of reserve material stored up in its cytoplasm, 

 is the female element and is called the ovum. The other cell, which consists 

 of little more than nuclear material, is the male element and is called the 

 spermatozoon. Both kinds of cells are derived from a mass of undifferentiated 

 cells, the germ epithelium which, as we have seen, can often be traced directly 

 back to the first divisions of the fertilised egg. The use of the reserve 

 material in the ovum is to serve as food for the developing individual. The 

 ovum 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 division the nuclei of all somatic 

 cells, whether of plants or animals, undergo a series of changes which, in 

 their broad outlines, are similar throughout both animal and vegetable 

 kingdoms (Fig. 568), and result in the production of qualitatively identical 

 daughter nuclei. 



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 cytoplasm 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 condition 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) Antiphase (the travelling of each half -chromosome to the extremity of the spindle). 



(4) TelopJiase (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 any cell is constant for the species. 

 Thus every human somatic cell has sixteen chromosomes in its nucleus. The same 

 number is found in types so far apart as the ox, the guinea-pig, and the onion. In 

 the mouse, the salamander, and the lily the number is twenty-four. Other types, such 

 as the crustacean Artemia, are said to have as many as 168 chromosomes, while in Ascaris 



