42 MANUAL OF ZOOLOGY. 



the perfect individual ; and the chief differences which are ob- 

 served in the process as it occurs in different animals consist 

 simply in the extent to which these changes are external and 

 visible, or are more or less completely concealed from view. 

 For these differences the terms " transformation " and " meta- 

 morphosis" are employed; but they must be regarded as 

 essentially nothing more than variations of development. : 



Transformation is the term employed by Quatrefages to 

 designate " the series of changes which every germ undergoes 

 in reaching the embryonic condition ; those which we observe 

 in every creature still within the egg ; those, finally, which the 

 species born in an imperfectly developed state present in the 

 course of their external life." 



Metamorphosis is denned by the same author as including 

 the alterations which are " undergone after exclusion from the 

 egg, and which alter extensively the general form and mode of 

 life of the individual." 



Though by no means faultless, these terms are sufficiently 

 convenient, if it be remembered that they are merely modifi- 

 cations of development, and express differences of degree and 

 not of kind. An insect, such as a butterfly, is the best illus- 

 tration of what is meant by these terms. All the changes 

 which are undergone by a butterfly in passing from the fecun- 

 dated ovum to the condition of an imago, or perfect insect, 

 constitute its development. The egg which is laid by a butter- 

 fly undergoes a series of changes which eventuate in its giving 

 birth to a caterpillar, these preliminary changes constituting 

 its transformation. The caterpillar grows rapidly, and after 

 several changes of skin becomes quiescent, when it is known 

 as a " chrysalis." It remains for a longer or shorter time in 

 this quiescent and apparently dead condition, during which 

 period developmental changes are going on rapidly in its 

 interior. Finally, the chrysalis ruptures, and there escapes 

 from it the perfect winged insect. To these changes the term 

 metamorphosis is rightly applied. These changes, however, do 

 not differ in kind from the changes undergone by a Mammal ; 

 the difference being that in the case of a Mammal the ovum 

 is retained within the body of the parent, where it undergoes 

 the necessary developmental changes, so that at birth it has 

 little to do but grow, in order to be converted into the adult 

 animal. 



From these considerations we arrive at a second generalisa- 

 tion, which is thus formulated by Quatrefages : " Those crea- 

 tures whose ova owing to an insufficient supply of nutritious 

 contents, and an incapacity on the part of the mother to pro- 



