6 ZOOLOGY. 



well as the entire Amoeba or monad, is all alike, being per- 

 fectly homogeneous to the chemist or microscopist of our 

 day. Yet potentially the protoplasm of different cells exerts 

 widely different forces and capabilities. An egg-cell be- 

 comes a man, whose brain-cells are the medium of the intel- 

 lectual 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 Amoeba or an Infusorian, are said to be 



O 7 



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 



division of the yolk into two, four, 

 eight, and afterward many cells ; the 

 cells thus formed become arranged into 

 two layers or sets called germ-layers. 

 The outer is called the ectoderm and 

 the inner the endoderm. A third germ- 

 layer arises between them, called the 

 Fig.3.-GenTof Sagitta. mesoderm or middle germ-layer. From 

 So?SSia#!SE these germ-layers, or cell-layers, the 

 cieated cells. tissues of the body are f ormed, such as 



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

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

 ganisms, 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- 



