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INTEODUOTION. 



The history of the development of the individual, or Ontogeny 

 (Embryology), is the science of the growth of an organism ; 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 Eoster and Balfouk 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 hook of that kind is especially suitable for such persons as 

 desire to acquaint themselves, by their own observation, with the 

 emhryology 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. 



For beginners, and for the needs of theoretical instruction ir 

 Embryology, the second method commends itself, in which the separate 

 organs are considered in succession, each for itself, and the changes 

 which a single organ has to pass through during development are 



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