ESSENTIAL FEATURES OF SEXUAL PROCESS 1205 



of progress in a race that we see in many civilised communities, where the 

 power and the conduct of affairs are in the hands of the older members. 



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 identical throughout both animal and vegetable 

 kingdoms (Fig. 555). The nucleus of the resting cell in its vegetative condi- 

 tion 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 chromo- 

 somes. Each of the filaments, in large nuclei, may often be seen to be com- 

 posed 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 them- 

 selves 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) Metapjiase (the splitting of the chromosomes). 



