74 INTRODUCTION TO 



evaporation of the 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 he proved hy 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 he 

 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. Suckow first 

 observed the intestinal canal, displaying even the 

 constrictions which separate the oasophagus 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 rigidit/ 



