CHAPTER n 



THE GAMETES 



DEVELOPMENT in sexually reproducing organisms is usually considered to 

 begin at the time when the sperm unites with the egg. But actually these 

 two types of cells are among the most complex formed in the animal body, 

 and themselves undergo very important processes of development before 

 they are ready to perform their characteristic functions. The undifferen- 

 tiated cells which will eventually give rise to them are collectively known 

 as gametocytes, and separately as oocytes if they will form ova, or sperma- 

 tocytes if they will form sperm. The fully differentiated cells are 

 known as gametes. The male type are called sperm, or spermatozoa, 

 both terms being correct and with exactly the same meaning. The female 

 type are referred to as eggs or ova, but these two words do not mean 

 quite the same thing. The word ovum refers strictly to the gamete-cell; 

 and this often makes up only a part of the body known as the egg, which 

 may include a number of membranes, layers of jelly, shell, etc., which 

 strictly speaking lie outside the ovum, and are no part of it. Thus in the 

 hen's egg, the ovum is only that part conventionally known as the 'yoh^'. 

 The basic functions of the gametes are, firstly, to bring together the two 

 nuclei contributed to the offspring by the parents, and secondly to carry 

 out the development of the new individual until it is fully enough 

 formed to take in its own nourishment. A good deal of preparatory 

 differentiation is required before an ordinary cell can be fitted for either 

 task. It is not appropriate here to discuss in any detail the preparation of 

 the gamete-nucleus, since this subject really belongs to the alhed discipline 

 of Genetics, and is fully described in textbooks of that subject. It is only 

 necessary to remember that, whereas the nucleus of a normal body cell 

 contains two of each kind of chromosome, and thus two of each kind 

 of gene, in the gamete-nucleus these are reduced to one representative of 

 each kind. The reduction takes place by a sequence of two divisions, 

 known as the meiotic, or maturation divisions (the term 'reduction 

 division' cannot strictly be apphed to either the first or second of these, 

 but only to both together). The matured gamete-nuclei, containing only 

 one of each sort of chromosome, are known as haploid, while the normal 

 condition is known as diploid. ^ 



^ In some organisms (many plants and a few animals) the body cells contain more than 

 two of each kind of chromosome, and are then said to be 'polyploid' ; in this case the 

 gametes, if they are formed at all regularly, contain half the number in the body cells, 

 and thus more than the haploid number; but many irregularities occur in such cases. 



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