EGGS OF INSECTS. 693 



they closely resemble each other while yet a part of the 

 ovarian ova, and prior to their detachment from the ovary. 

 At one period of their formation all eggs consist of three 

 similar parts : — 1st. The internal nucleated cell, or ger- 

 minal vesicle, with its macula ; 2d. The vitellus, or yolk- 

 substance ; and 3d. The vesicular envelope, or vitelline 

 membrane. The germinal vesicle being the first produced 

 may be regarded as the ovigerm ; then comes the yolk- 

 substance, which gradually envelopes it, or is deposited 

 around it ; and the vitelline membrane, the latest formed, 

 incloses the whole. The chemical constituents of the ego: 



DO 



contents are albumen, fatty matters, and a proportion of a 

 substance precipitabie by water. The production of the 

 chorion, or shell membrane, does not take place till the 

 ovum has attained nearly the full size, and it then appears 

 to proceed, in part at least, from the consolidation over the 

 whole surface of one or more layers of an albuminous fluid 

 secreted from the wall of the oviduct. The observations 

 of H. Meyer have shown that a part of the outer mem- 

 brane is derived from a conversion into it of the inner 

 cellular or epithelial lining membrane of the oviduct, at 

 the place where it is in closest contact with the surface of 

 the ovum. Dr. Allen Thompson therefore thinks that 

 " many of the varieties in the appearance and structure of 

 the external covering of eggs may probably depend on the 

 different modes of development of these cells." 



The embryo cell is so directly connected with the 

 germinal vesicle that at a certain period it disappears alto- 

 gether, and is absorbed into the germinal yolk, or rather 

 becomes the nucleus of the embryo, when a greater degree 

 of compactness is observed in the yolk, and all that remains 

 of the germinal vesicle is one or more highly refracting 

 fat globules and albuminoid bodies. Towards the end of 

 the period of incubation, the head of the young cater- 

 pillar, according to Meissner, lies towards the dot or 

 opening in the lid, which he terms the micropyle? from its 

 resemblance to a small gate, or opening through which the 

 worm emerges forth. From a number of careful observa- 



L o v 



(1) The term micropyle (a little gate) has heretofore only been used in its re- 

 lation with the vegetable kingdom : it is used to denote the opening or Coram t- a 

 towards which the radicle is always pointed. 



