CHAPTER XII 



THE MINUTE STRUCTURE OF CELLS : CELL-DIVISION : 

 STRUCTURE OF THE OVUM : SPERMATOGENESIS AND 

 OOGENESIS : MATURATION AND FERTILIZA1 ION OF THE 

 OVUM I SEGMENTATION OF THE OOSPERM : EFFECT OF 

 FOOD-YOLK ON DEVELOPMENT : FORMATION OF THE 

 CHIEF ORGANS OF VERTEBRATES, AND OF THE AMNION, 

 ALLANTOIS AND PLACENTA. 



Structure of the Cell. We have learnt in previous chap- 

 ters that all organisms are formed essentially of one or more 

 cells (compare e.g., pp. no and 1 18) ; 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 (p. 106). 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 an optical section of it that is, all the parts which 



