6 ZOOLOGY. 



well as the entire Amoeba or monad, is complex. It is com- 

 posed of carbon, hydrogen, oxyg6n, and nitrogen, combined 

 in nearly the same proportions. The protoplasm of diffei-ent 

 cells exerts widely different forces and capabilities. An egg- 

 cell becomes a man, whose brain-cells are the medium of the 

 intellectual power which enables him to write the history of 

 his own species, and to be the historian of the forms of life 

 which stand below him. The cell is the morphological 

 unit of the organic world. With cells the biologist can 

 in the imagination reconstruct the vegetable and animal 

 worlds. 



The primitive form of a cell, when without a nucleus or 

 nucleolus, is called a cytode ; genuine cells have a nucleus, 

 the latter containing a nucleolus. Animals composed of but a 

 single cell, such as the Amceba or an Infusorian, are said to be 

 unicellular. Cells grow by absorbing cell-food — i.e., by the 

 assimilation of matter from without, and this matter may be 

 in masses of considerable size when seen under the microscope. 

 Cells multiply by self-division. The egg-cell undergoes 

 ^^-ry-p>-. division of the yolk into two, four, 



XV^JuX/avN eight, and afterward many cells ; the 

 ^'■/^^^^'^t^^^^l cells thus formed become arranged into 



cSffif v%vr^ two layers or sets called germ-layers. 



r^ra, ; ffl^Q '^^^ outer is called the ectoderm and 



N^^gi ! (^^v *^® inner the endoderm. A third germ- 

 xiSx'J'xlV^ layer arises between them, called the 

 Fig. 3.-Genn of sagitta. mesoderm or middle germ-layer. From 

 foth^t'e^feed'ofe these germ-layers, or cell-layers, the 

 cieated cells. tissues of the body are formed, such as 



muscle, bone, nerve, and glandular tissue. These tissues 

 form organs, hence animals (as well as plants) are called or- 

 ffanisms, because they have certain parts formed of a partic- 

 ular kind of tissue set apart for the performance of a special 

 sort of work or physiological labor. This separation of 

 parts for particular or special functions is called differentia- 

 tion ; and the highest animals are those whose bodies are 

 most differentiated, while the lowest are those whose bodies 

 are least differentiated ; hence high animals are specialized. 

 and, on the other hand, low animals are simple. Thus dif- 



