AND THE LOWER ANIMALS. 113 



measures 20 centimetres in length, and has attained 

 a weight of 224 grammes. In the course of four 

 months, then, it has become thirty times as long and 

 eighteen hundred times as heavy as it was when we 

 first examined it. From this time forward it grows 

 about an inch, or nearly three centimetres, every 

 fifteen days ; till, at the time of its birth, it has 

 attained a length of nearly half a metre, and a weight 

 of about three and a half kilogrammes.* Thus we 

 see that in less than nine months the human embryo 

 has become, in round numbers, seventy times longer 

 and twenty-nine times heavier than it was when three 

 weeks old. 



Among insects we find an analogous state of things. 

 According to Redi, the larva of a meat-fly (Musca 

 carnaria) becomes, in the short space of twenty-four 

 hours, from a hundred and forty to two hundred times 

 heavier than it was before. f Lyonnet has shown, by 

 direct observation, and by calculation, that the cater- 

 pillar we spoke of before (Cossus ligniperda) is 

 seventy-tivo thousand times heavier in the chrysalis 

 condition than when it emerged from the egg» 



Insects do not generally increase in size as they 

 reach the perfect condition, but, on the contrary, are 

 very much smaller than when in the larval state. This 

 decrease in size is particularly well marked in the 

 Stratiomydce; whose history we have given in another 

 chapter ; but these insects may be regarded as excep- 



* In giving these dimensions of the embryo at different periods 

 of its existence, I have quoted the figures of Olivier and Chaussier, 

 whose results were arrived at by an examination of fifteen thousand 

 specimens. — Dictionnaire de Medecine. 



t " Introduction a l'Entomologie," by Th. Lacordaire. 



I 



