by special sex or germ cells) where the nucleus of the male 

 cell or spermatozoon unites with that of the female cell 

 or ovum. As seen in the resting state (i.e., before division 

 takes place) it is a body of rounded shape, and it appears 

 to be enclosed in a very delicate membrane. Its contents 

 partly consist of linin, which forms a kind of network, 

 and is a substance difficult to stain. Amidst the linin are 

 many granules of chromatin, a very complex nitrogenous 

 compound which always contains phosphorus and has a 

 marked affinity for certain stains. 



Most nuclei contain one or more smaller bodies known 

 as nucleoli, and these are probably reserves of complex 

 materials which can be drawn on at certain periods of 

 nuclear activity, for it is at such times that the nucleolus 

 exhibits great change or wholly disappears. In some cells 

 the nuclear material is scattered throughout the cytoplasm 

 and does not appear as a compact nucleus. 



Describe the Process of Cell Division. 



The multiplication of the ordinary uninucleate somatic 

 or body cells is brought about by division (fission). Di- 

 vision of the cell is preceded by division of the nucleus, 

 which in some instances is effected by simple constriction, 

 and this method is described as direct (amitosis). In most 

 cases the division is more complicated, and involves re- 

 arrangement of certain nuclear constituents. That method 

 is indirect (mitotis or karyokinesis), and the phenomena 

 are as follows : 



The centrosome (if only one is present) divides into two. 

 These go apart to opposite poles of the nucleus, and from 

 each, and from its surrounding archoplasm, a number of 

 fibrils radiate out and extend into the cytoplasm. This is 

 the aster stage, the figures being the star-like astrospheres or 

 attraction spheres. Meanwhile in the nucleus the chromatin 

 has become a thickened thread skein or spireme, and this 

 breaks up into segments, called chromosomes, the number 

 of which is definite and constant in each species of animal. 

 Then the nuclear membrane disappears, and a group of 



