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 Malpighi 
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 (Mémoires pour 
servir a Vhistoire des polypes deau 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. We have seen 
that Swammerdam, in the seventeenth century, had begun 
observations upon the anatomy of tadpoles, frogs, and snails, 
