PROGRESS OF MINUTE ANATOMY 103 



sensory cells are also modifications of surface cells, and, as 

 a preliminary step to understanding their particular office, 

 we must know the line along which they have become modi- 

 fied to fit them to receive stimulation. 



Then, if we attempt to follow in the imagination the way 

 by which the surface stimulations reach the central nervous 

 system and affect it, we must investigate all the connections. 

 It thus appears that we must know the intimate structure of 

 an organ in order to understand its physiology. Leydig 

 supplied this kind of information for many organs of insects. 

 In his investigations we see the foundation of that delicate 

 work upon the microscopic structure of insects which is still 

 going forward. 



Summary. — In this brief sketch we have seen that the 

 study of insect anatomy, beginning with that of Alalpighi 

 and Swammerdam, was lifted to a plane of greater exactitude 

 by Lyonet and Straus-Diirckheim. It was further broadened 

 by the researches of Dufour, and began to take on its modern 

 aspects, first, through the labors of Newport, who introduced 

 embryology as a feature of investigation, and, finally, through 

 Leydig's step in introducing histology. In the combination 

 of the work of these two observers, the subject for the first 

 time reached its proper position. 



The studies of minute structure in the seventeenth and 

 eighteenth centuries were by no means confined to insects; 

 investigations were made upon a number of other forms. 

 Trembley, in the time of Lyonet, produced his noteworthy 

 memoirs upon the small fresh-water hydra (Memoires pour 

 servir a Phisloire des polypes (Teau douce, 1744); the illustra- 

 tions for which, as already stated, were prepared by Lyonet. 

 The structure of snails and other mollusks, of tadpoles, frogs, 

 and other batrachia, was also investigated. AVe have seen 

 that Swammerdam, in the seventeenth century, had begun 

 observations upon the anatomy of tadpoles, frogs, and snails, 



