1 have divided the text into two parts, in order 

 that the more general facts may be separated from 

 the more special ; and I have further had the 

 most technical portions of the subject printed in 

 smaller type. The correct appreciation of homo- 

 logies has been my aim throughout the entire 

 work, and the whole has been drawn up from my 

 own dissections. I have made no statements 

 merely upon the authority of others, except in 

 one or two instances, in which this is specially 

 stated. 



Dr. Weismann's elaborate researches on the 

 Development of the Diptera have greatly aided me; 

 and my best thanks are due to Mr. C. Stewart, 

 of St. Thomas's Hospital, for the kind manner 

 in which he has allowed me access to his 

 cabinet, and the readiness with which he has 

 always aided me by his opinion or advice. 



I have to acknowledge also the kindness of 

 Mr. Curtics for the aid he has afforded me in. 

 bringing out the present Work. 



