DEVELOPMENT. 35 



Transformation is the term employed by Quatrefages to de- 

 signate " the series of changes which every germ undergoes in 

 reaching the embryonic condition j 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 defined 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 fe- 

 cundated 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 giv- 

 ing birth to a caterpiUar, these preliminary changes constitut- 

 ing 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 the second law laid 

 down by Quatrefages : — " Those creatures whose ova — owing to 

 an insufficient supply of nutritious contents, and an incapacity 

 on the. part of the mother to provide for their complete de- 

 velopment within her own substance — are rapidly hatched, 

 give birth to imperfect offspring, which, in proceeding to their 

 definitive characters, undergo several alterations in structure 

 and form, known as metamorphoses." 



Retrograde Development. — Ordinarily speaking, the course 

 o'f development is an ascending one, and the adult is more 

 highly organised than the young ; but there are cases in which 



