338 PHYSIOLOGY. 



The separate albumen which is observed in the eggs of the Mollusca, 

 Arachnida, Crustacea, many fish, and the Amphibia, and birds, is 

 therefore wholly wanting in the eggs of insects, which consist solely 

 of yolk. 



We have as yet but little information of the progress of the formation 

 of the embryo from this fluid; we only know from Suckow's * observa- 

 tion in Gastrophaga pini that a small dark spot is formed in the centre 

 of the originally tolerably clear yolk, which he considers as the com- 

 mencement of the embryo. From this point, which we prefer consider- 

 ing upon the surface of the yolk analogously to the development of other 

 animals, and not as would appear from Suckow's observation in its 

 middle, the formation of the embryo so proceeds that the ventral surface 

 along which the nervous cord runs first presents itself. This ventral 

 plate distends on all sides, gradually growing completely over the yolk, 

 which is thereby enclosed completely within the ventral cavity. This mode 

 of development has not yet indeed been observed in true insects, but the 

 development of the Crustacea and of the Arachnida speaks in favour 

 of it. After a short period the embryo appears distinctly as a half 

 moon-shaped body, at the end of which the head is already perceived 

 (PI. I. f. 24. A.). The embryo swims in a bright green but clear fluid, 

 the liquor amnii, and it is enclosed by two other membranes besides the 

 shell. The innermost, the amnion, which contains the water, is spongy, 

 and exhibits upon its inner surface small glands that are surrounded by 

 a bright margin, arid it is covered exteriorly by a cluster of webbed vessels 

 (the same, c, c, c), which all proceed from a thicker main stem, which 

 opens into the orbicular portion of the egg filled with air. These 

 vessels, which doubtlessly convey air, consist, according to Suckow, of 

 but a single transparent membrane, and therefore differ considerably 

 in structure from true tracheae. Michelotti's t experiments upon the 

 eggs of Liparis dispar and L. mori have proved that the eggs, during 

 their development, decompose air, viz., imbibe oxygen, and give out 

 carbonic acid, but only in a temperature of from 15 to 20, whereas 

 beneath zero they leave the atmospheric air unaltered. This absorption 

 of oxygen is necessary to their development, for the eggs speedily die 

 in miasmatic gases, which are free from it. If now, as appears neces- 



* See his Anatomisch. Physiologisehen Untersuchungen der Insekten uml Krustcn- 

 thiere, vol. i. part i. Heidelb. 1818. 4to. 



f- See PfafF and Friedlandcr franzosische Annalcn, part iv. p. 48, &r. 



