Page:Jardine Naturalist's library Entomology.djvu/80

74 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 calcarious 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, enclosing the fluid within, the whole of which must be regarded as the yolk. The latter is a thick granulated 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 detected in the process of development. Suckow first observed the intestinal canal, displaying even the constrictions which separate the œsophagus and intestine 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 Swammerdam observed its distinct pulsations. The commencement of the nervous system appears in two scarcely perceptible filaments, which gradually approximate, 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