CHAFPER II 



Structure of a cell Phenomenon of cell division Selective division of certain 

 parts Theory identifying Nageli's idioplasm with chromatin Objections 

 The maturation of the gametes The meiotic phase The impossibility of 

 particular characters common to the race being borne by particular chromo- 

 somes Fertilisation Parthenogenesis Fertilisation in \iuicellular animals 

 Experiments in artificial fertilisation and their bearing upon the existence 

 of an hereditary substance Conclusions. 



WE have already seen that a cell consists of a minute mass 

 of protoplasm. Within this mass is a denser area surrounded 

 by a membrane the nucleus. The rest of the cell sur- 

 rounding the nucleus is called the cytoplasm (Fig. 1). In 

 the cytoplasm, generally near the nucleus, two minute bodies 

 may frequently be observed. These are known as the centro- 

 somes (Fig. I). 1 When a cell is in the vegetative condition, 

 that is, when it is not preparing to divide, or in process of 

 division, the nucleus is seen to contain numerous small 

 masses of a substance which is denser than the rest of the 

 contents of the nucleus. These small masses are joined 

 together by a fine meshwork, which frequently contains 

 still smaller particles of this same substance. When cells 

 are stained in order to make them more easy to study under 

 the microscope, this dense substance, joined together by the 

 line meshwork, stains more darkly than any of the other 

 parts of the cell. For this reason it has been called chromatin. 

 The chromatin is contained in an envelope of clear slightly 

 staining material, which is known as linin. The fine mesh- 

 work joining the masses of chromatin is also formed of linin, 

 and granules of chromatin are distributed in the strands. 

 When a cell is going to divide into two daughter cells, 



1 Ccntrosomes are not present in the cells of the higher plants. (See note at 

 end of chapter. ) ,. 



