The cell membrane, starch grains, pigment granules, 

 oil globules, chlorophyll granules and protein granules. 



(Fig. 22). 



New cells arise by a process of division from the mother 

 cell, which divides into two cells, and each of these into two 

 cells, and so the cells multiply until the body of the plant or 

 animal is constructed. This original mother-cell is called 

 the fertilized egg-cell or zygote, which as we have already 



cm 



Fig. 22. — A, ovarian ovum of cat; cm. cell mem- 

 brane; mi, microsomes; o, nucleolus; nm, nuclear 

 membrane; y. yolk alveoli. B, cell from root cap of 



callalily; d, plastids. 



noted, arises by the fusion of two independent cells— the 

 gametes or germ-cells — male and female. The female 

 gamete is called the egg-cell or ovum, the male gamete, the 

 sperm-cell, spermatozoon or spermatozoid. (See also 

 page 43). 



Nuclear Division. — In the division of cells the nucleus 

 plays a prominent part. On staining the cell the nucleus 

 is seen to have a complex structure. It is composed of a 

 network of fibrils, enclosing in its meshes a clear substance. 

 Embedded in the fibrils are granules of a substance which 

 stains very deeply, called chromatin. When a cell is about 

 to undergo division the chromatin substance changes from 

 a network to a tangled thread which gradually shortens and 

 thickens and breaks up into a number of rods called chromo- 

 somes. The nuclear membrane disappears about this time. 

 Then appear a number of delicate fibrils, arranged in the 

 form of a spindle, and the chromosomes arrange them- 

 selves along the equator of the spindle. They are 



1.30 



