INTRODUCTION. 



EVERY living being passes in the course of its life through a 

 series of changes of shape and structure. These changes 

 may, in their completest form, be considered as constituting 

 a morphological cycle, beginning with the ovum and ending 

 with the ovum again. 



Among many living beings and especially among verte- 

 brate animals by far by the greater part of the life of the 

 individual is spent in one particular phase, which is not only 

 of longer duration than the rest, but also of much more 

 importance, inasmuch as during it the greater part of the 

 ' work ' of the living being is done. This is generally spoken 

 of as the adult stage, and in most cases immediately precedes, 

 or is peculiarly associated with, the completion of the morpho- 

 logical cycle in the appearance of a new ovum. 



The word embryology may be generally taken to mean 

 the study of the successive morphological phases through 

 which a living being passes from the ovum to the adult 

 stage, or the study of the gradual 'development' of the ovum 

 to the adult form ; though, especially among some of the so- 

 called lower forms of life, its meaning must be so extended 

 as to embrace all the morphological phases of an individual 

 life. Embryology is thus a part of and a necessary intro- 

 duction to the wider study of ' Generation.' As a matter of 



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