OF GENERATION. 337 



If we take a retrospection of the whole process of the development 

 of the germ to the egg we shall find that there are three distinct periods 

 in its progress. The filiform superior appendage of the egg-tube is 

 the first, for in it takes place the secretion of the formative matter, and 

 from here it descends into the egg-tube as a germen. The remainder, 

 probably albuminous portion, of the secretion, remains, as placentula, 

 between every two egg germs. The second period is the loosening of 

 the placentula by copulation. By means of it the internal tunic comes 

 into close contact with the exterior vascular one, in consequence 

 of which the ring is formed ; and at the same time the impregnation of 

 the germ takes place by the male semen imbibed from the placentula. 

 The ring, lastly, is the third period ; it promotes, by supplying the 

 placentula with atmospheric air, its capacity of appearing as a new 

 organic mass, so that it may be gradually imbibed by the growing egg. 

 The yolk thus becomes perfectly formed, and envelopes itself with its 

 second tunic, and then with its shell, which is hardened also by means of 

 the air from the ring. The formation of the egg is then completed, and 

 the period of laying comes, which takes place immediately, to make 

 room for a still immature egg. It is from this circumstance that some 

 insects, namely, those with many egg tubes, for example, the queen bee, 

 require a long time to lay all their eggs, and only in those with bag and 

 bladder-shaped ovaries, which are furnished upon their surface with 

 short egg-tubes (as, for example, Lytia and Meloe,} can the eggs be 

 almost all matured at the same time. 



211. 



When, after all this procedure, the egg has quitted the maternal 

 sphere, a distinct life, namely, that of the embryo, commences in it. 

 If we first survey the structure of the laid egg we shall observe that it 

 consists externally of a horny shell, which becomes tolerably hard in 

 the air, and is in general transparent or colourless, but less frequently 

 decorated with particular markings and colours. Beneath this external 

 covering lies a second, finer, more delicate membrane, which forms the 

 case of the fluid contained within the egg. This fluid is the yolk, 

 (vitellus,} a yellow, whitish, or green, thick, granulated mass, which in 

 Pkasma is dotted with purple, and it chemically consists of albumen, 

 some animal glue, a yellow fat oil and sulphate and phosphate of natron *. 



* See John's Cheiuische Schrift, vol. ii. p. 112. 

 Z 



