I02 BIOLOGY AND ITS MAKERS 
Von Baer had founded embryology by his great work on the 
development of animals in 1828, before the investigations of 
Dufour, but it was reserved for Newport to recognize its 
great importance and to apply it to insect anatomy. He saw 
clearly that, in order to comprehend his problems, the anat- 
omist must takeinto account the processof building the body, 
as well as the completed architecture of the adult. The in- 
troduction of this important idea made his achievement a 
distinct advance bevond that of his predecessors. 
Leydig.—Just as Newport was publishing his conclusions 
the cell-theory was established (in 1838-39); and this was 
destined to furnish the basis for a new advance. The in- 
fluence of the doctrine that all tissues are composed of similar 
vital units, called cells, was far-reaching. Investigators began 
to apply the idea in all directions, and there resulted a new 
department of anatomy, called histology. The subject of 
insect histology was an unworked field, but manifestly one 
of importance. Franz Leydig (for portrait see p. 175) 
entered the new territory with enthusiasm, and through his 
extensive investigations all structural studies upon insects 
assumed a new aspect. In 1864 appeared his Vom Bau des 
Thierchen Korpers, which, together with his special articles, 
created a new kind of insect anatomy based upon the micro- 
scopic study of tissues. The application of this method of 
investigation is easy to see; just as it is impossible to under- 
stand the working of a machine without a knowledge of its 
construction, so a knowledge of the working units of an organ 
is necessary to comprehend its action. For illustration, it is 
perfectly evident that we can not understand what is taking 
place in an organ for receiving sensory impressions without 
first understanding its mechanism and the nature of the 
connections between it and the central part of the nervous 
system. The sensory organ is on the surface in order more 
readily to receive impressions from the outside world. The 
