Excerpt from Text-Book of the Embryology of Man and Mammals

The history of the development of the individual, or Ontogeny (embryology), is the science of the growth of an organism 5 it de scribes the morphological changes which an organism passes through from its origin in the ovum up to its complete maturity, and presents these in their natural connection. We can regard the fertilisation of the egg-cell as the beginning of the process of development for Vertebrates, as it also is for all the rest of the higher animals.

In giving an account of the changes of the egg-cell, which begin with fertilisation, one may choose between two different methods.

According to one method a particular organism is made the basis of the account, and one describes the changes which its germ under goes from the moment of fertilisation onward, from hour to hour, and from day to day. It is in this way that the embryology of the Chick has been worked out by C. E. Von baer in his classical paper, and by foster and balfour in their Elements of Embryology. This method has the advantage that the reader acquires a View of the total condition of an organism in the separate stages of its development.

A book of that kind is especially suitable for such persons as desire to acquaint themselves, by their own observation, with the embryology Of a single animal, as, for example, the Chick, by repeating the investigations of others. It is, on the contrary, less adapted to those who wish to acquire a connected view of the development of the separate organs, as the eye, the heart, the brain, etc. For the formation of these will of course be treated of at different places in describing younger and Older embryos. In order to procure a general survey of the course of development of an organ, the reader must consult various places in the text-book, and collect for himself what relates to the subject.

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