74 
INTRODUCTION TO 
evaporation of tlie fluids ; the latter is exemplified by 
such as are exposed (as they often are among the 
Lepidoptera) to the action of the elements. No cal- 
carious ingredient enters into the composition of the 
envelope, as may be proved by the application of an 
acid which produces no effervescence. Under the 
outer envelope there is another very thin pellicle, en- 
closing the fluid within, the whole of which must be 
regarded as the yolk. The latter is a thick granul- 
ated mass, variable in colour, and, as far as it has 
been examined, found to consist of albumen, some 
animal glue, a yellow oil, sulphate and phosphate of 
natron. With the earlier stages of embryo life, we 
are still imperfectly acquainted ; but when it has 
continued for some time, several organs can be de- 
tected in the process of development. Suekow first 
observed the intestinal canal, displaying even the 
constrictions which separate the (esophagus and in- 
testine from the stomach. Air-vessels are likewise 
visible, but their function is as yet dormant, as 
they have no communication with the atmosphere. 
The dorsal vessel is also developed, and Swammer- 
dam observed its distinct pulsations. The com- 
mencement of the nervous system appears in two 
scarcely perceptible filaments, which gradually ap- 
proximate, till they unite at different points forming 
incipient ganglia. The head, mandibles, and anal 
horn, (when the latter exists,) are, in general, the 
parts most distinctly marked in embryos. 
It is a very anomalous fact, that the eggs of insects 
often augment in bulk after they are laid. The rigidity 
