THE CELL DOCTRINE. 91 



spinal cord to the extremity of the foot. Their di- 

 ameter is, however, small, often not exceeding .001 

 millimetre, or .00003987 of an inch. 



The tubes offer as objects of study the walls and 

 the cavity. 



The cells of vegetables have a wall, a cavity, and 

 contents (air, oil, &c.). The cells of 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 re- 

 actions 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 parti- 

 cles 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 simultaneously. In the 

 same way, the division takes place into 4, 8, 16, and 

 more parts. These divisions of the vitellus have re- 

 ceived the name vitelline globules. Their mode of 

 formation is called segmentation. 



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



