10 Dynamic Theory. 



new animal composed of a single cell with its nucleus. All animals of 

 every grade are composed almost entirety of nucleated cells, and the 

 most of these cells have the same mode of growth, and increase in num- 

 ber in the same manner as the protozoan. 



FIG. 8. FIG. 9. 



FIG. 8. Amoeba much enlarged "processes" extended as in the act of creeping, nu- 

 cleus and nucleolus in center. 



FIG. 9. Egg cell of chalk sponge (Olyn thus). It moves about within the sponge and 

 is in all respects like the amoaba. (Haeckel.) 



The cells of all the higher grades of animals are differentiated in 

 various ways and are not homogeneous as they are in the case of the 

 lowest animals. Thus, one class of cells composes the skin, another 

 class the bone, another the blood, another the nerves, another the repro- 

 ductive eggs, &c. These are ail similar in their fundamental forms 

 and chemical composition, and yet they are devoted to different ends 

 and functions. 



In the protozoan, the single cell is by turns a stomach, a locomotive 

 apparatus, and an egg. In the higher animals only one set of cells act 

 as eggs, while the functions of locomotion, digestion, &c. , are per- 

 formed by organs composed of other classes of cells. The growth of 

 the eggs and the subsequent development of the embryo, have been 

 closely watched, and comparisons have been made, of the mode of this 

 growth amongst all classes of animals. The comparison between man 

 and the other mammals, shows that the same principles, with certain 

 progressive differences of detail, govern the development of the embryo 

 throughout the whole class, and that the difference in the detail of the 

 development is only such as might be expected to occur in view of the 

 different conditions of the lives of the parent animals. 



There is no distinction or differentiation of sexes among the protozo- 

 ans, and some other grades of simple animals, but each animal unites 

 the sexual qualities of both sexes in its one cell amongst the other mul- 

 tifarious qualities of that cell. But in all except the lowest animals 

 (and plants too) the male and female principles are differentiated and 

 one set of cells is set apart for the embodiment of the male principle, 

 and another for the female. And before the embryo can be formed, a 

 male and a female cell must coalesce and the two unite their protoplasm 



