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168 is essentially different from a muscular fibre; while, on the other hand, a whole primitive muscular fasciculus is identical in its mode of origin with a nervous fibre, and so on. The existence of a common principle of development for all the elementary parts of organic bodies lays the foundation of a new section of general anatomy, to which the term philosophical might be applied, having for its object—firstly, to prove the general laws by which the elementary parts of organisms are developed; and, secondly, to point out the different elementary parts in accordance with the general principle of development, and to compare them with one another.

The foregoing investigation has conducted us to the principle upon which the elementary parts of organized bodies are developed, by tracing these elementary parts, from their perfected condition, back to the earlier stages of development. Starting now from the principle of development, we will reconstruct the elementary parts as they appear in the matured state, so that we may be enabled to take a comprehensive view of the laws which regulate the formation of the elementary particles. We have, therefore, to consider—1, the cytoblastema; 2, the laws by which new cells are generated in the cytoblastema; 3, the formative process of the cells themselves; 4, the very various modes in which cells are developed into the elementary parts of organisms.

Cytoblastema.—The cytoblastema, or the amorphous substance in which new cells are to be formed, is found either contained within cells already existing, or else between them in the form of intercellular substance. The cytoblastema, which lies on the outside of existing cells, is the only form of which we have to treat at present, as the cell-contents form matter for subsequent consideration. Its quantity varies exceedingly, sometimes there is so little that it cannot be recognized with certainty between the fully-developed cells, and can only be observed between those most recently formed; for instance, in the second class of tissues; at other times there is