THE CELL DOCTRINE. 99 



The cells of vegetables have a M'all, a cavitj', and 

 contents (air, oil, etc.). The cells oi animals, on the 

 contrary, are, as a rule, homogeneous. Animal cells 

 containing a cavity are only found exceptionally. 

 The substance of cells is ordinarily granular. Most 

 cells contain an ovoid nucleus more granular than 

 the substance itself. 



In all cells the nuclei afford different chemical 

 reactions from those of the substance of the element. 

 Each cell is an independent organism, passing through 

 various stages of development, from birth to death. 



The birth (origin) of the elements takes place by 

 1st, segmentation; 2d, genesis; 3d, epigenesis ; 4th, 

 germination. 



1st. Segmentation. — The human ovum is a small 

 hollow sphere, containing in its interior the vitellus 

 or yolk, which consists of granular matter in a hya- 

 line substance. At the end of a certain time par- 

 ticles of the granular matter approximate, unite, and 

 form a nucleus in the vitellus. Next, the nucleus 

 elongates, takes an hour-glass form (biscuit), then 

 divides. The division of the yolk occurs simulta- 

 neously. In the same way, the division takes place 

 into 4, 8, 16, and more parts. These divisions of the 

 vitellus have received the name vitelline globules. 

 Their mode of formation is called segmentation. 



2d. Genesis. — "When the vitelline globes have be- 

 come very small by successive segmentation (diame- 

 ter .008 millimeter, .00031396 of an inch), these little 

 bodies take the name of embryonic cells. 



According to M. Eobin, these cells dissolve. From 

 the fusion results a blastema, in the midst of which 



