THE EGG ESSENTIALLY A CELL. 



15 



toplasm, and that out of this " primitive indifferent tissue^" 

 as it has been called, all the cells and tissues, and parts of 

 the animal body are evolved. 



The crjrj or ovum of all animals, even of the higliest, is essentially a 

 cell, and may be taken as the type of all cells. From this cell all the 

 other cells and parts of the animal body are developed ; the peculiar 

 process called segmentation of the yolk being the beginning of the for- 

 mation of new cells within the parent cell or ovum. 



It must be added here, however, that at its very beginning the ovum 

 or egg itself is something even much simpler than a cell, being merely 

 a minute particle of fluid matter, that is, mere protoplasm. 



Animals and plants are called Organic bodies or Organ- 

 ized beings, because they are made up of tissues, as ex- 

 plained above ; while all mineral substances as crystals, 

 earth, water, and gases are called Inorganic bodies, be- 

 cause they have not this structure. 



Chemically speaking, it may be stated in general terms 

 that animal tissues are mainly composed of carbon, hydro- 

 gen, oxygen and nitrogen ; while the first three elements, 



of small granules floating in it, and which fills the space between the nu- 

 cleus and the lining membrane or primordial utricle, already mentioned. 

 See Gray's Structural and Systematic Botany. 



FIG. 11. 



Various phases of the development of a unicellular plant, Palmoglwa macrococca; a 

 plant which spreads as a green slime over damp walls, etc. A, a full-grown cell ; B, 

 C, D, E, successive stages of one cell dividing into two. 



