CHAPTER XI 



THE MINUTE STRUCTURE OF CELLS : CELL-DIVISION : 

 STRUCTURE OF THE OVUM ; SPERMATOGENESIS : MATU- 

 RATION AND FERTILIZATION OF THE OVUM : SEGMEN- 

 TATION OF THE OOSPERM : EFFECT OF FOOD-YOLK ON 

 DEVELOPMENT : FORMATION OF THE CHIEF ORGANS OF 

 THE ADULT VERTEBRATE, AND OF THE AMNION, ALLAN- 

 TOIS, AND PLACENTA. 



Structure of the Cell. — AVe have learnt in previous chap- 

 ters that all organisms are formed essentially of one or more 

 «/Zf (compare pp. no and ii8); that the cell consists of 

 protoplas7it, and contains a nucleus consisting of chromatin 

 and achromatin (p. 129), in which one or more nucleoli 

 can usually be distinguished ; and that cells multiply by a 

 process of binary fission (p. 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 protoplasm 

 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 iin optical section of it— that is, all the parts which 



