CHAPTER XII 



THE MINUTE STRUCTURE OF CELLS I CELL-DIVISION T 

 STRUCTURE OF THE OVUM I GAMETOGENESIS (SPERMATO- 

 GENESIS AND OOGENESIS) I MATURATION AND FERTILISA- 

 TION OF THE OVUM I DIFFERENT TYPES OF OVA AND 

 OF SEGMENTATION EFFECT OF FOOD-YOLK ON DE- 

 VELOPMENT 



Structure of the Cell. We have learnt in previous 

 chapters that all organisms are formed essentially of one 

 or more cells (compare e.g. pp. no and 118) ; that the cell 

 consists of protoplasm, and contains a nucleus, enclosing 

 chromatin and usually one or more nucleoli (p. 129) ; and 

 that cells multiply by a process of binary fission (pp. 106, 

 236). It will now be necessary to study the structure 

 of a typical animal-cell and of its mode of division in 

 somewhat greater detail. 



There seems to be a good deal of variation in the 

 precise structure of various animal- and plant-cells, but 

 the more recent researches show that in the cell-body or 

 cytoplasm two constituents may be distinguished a clear 

 semi-fluid substance, traversed by a delicate sponge- 

 work. Now under the microscope the whole cell is not 

 seen at once, but only an optical section of it that is, all 

 the parts which are in focus at one time (see e.g. 



558 



