20 GENERAL EMBRYOLOGY 



tion, processes often described separately under the term 



histogenesis. 



This brief outline reflects the fundamental character of the 

 relation of Embryology, as of all biological science, to the Cell 

 Theory. The recognition of the ovum (Schwann, 1839; 

 Gegenbaur, 1861) and spermatozoon (Schweigger-Seidel, 1865) 

 as modified cells, of the basic importance of cell continuity in 

 development (Virchow), and of the processes of fertilization, 

 cleavage, growth, and differentiation as essentially cell pro- 

 cesses, marked noteworthy and fundamental steps in the 

 history of the science of development. But our recognition 

 of the importance of this relation, and of the especial importance 

 of the cell as the descriptive unit in development, should not 

 obscure the fact that in many developmental processes we 

 cannot recognize the cell as the actual unit of physiological 

 activity. Many important steps in development concern ele- 

 ments which are distinct and individual intra-cellular elements. 

 And later, during the cleavage period, the boundaries of 

 specific materials behaving as units in development do not 

 always coincide with cell boundaries or distributions. We 

 must regard the view that the cells are the ultimate units in 

 development as a stage in the history of opinion, and for the 

 present recognize certain intra-cellular elements as the "ulti- 

 mate" structures in development. 



But the province of Embryology is not merely thus to de- 

 scribe the upbuilding and unfolding of the structure and form 

 of the new organism through these successive stages of develop- 

 ment; it is, further, to describe the more fundamental pro- 

 cesses involved in this development, and still further, to 

 summarize these descriptions of both kinds in the form of simple 

 general statements or laws. In the historical development of 

 the science of Embryology, as of any natural science, the descrip- 

 tion and comparison of visible forms and conditions came 

 first. This morphological account of development, concerned 

 chiefly with the description of what happens, what is produced 

 in development, has now been accomplished to such an extent 



