324 Insects' Eggs. 



vesicle, with its macula. Second, The vitellus or yolk-substance; 

 and Third, The vesicular envelope or vitelline membrane. The 

 germinal vesicle is first produced, and may be regarded as the 

 ovigerm ; the yolk-substance next gradually envelopes it, or is 

 deposited around the germinal vesicle, and the vitelline mem- 

 brane which encloses the whole, is the latest formed. 



The chemical constituents of the ovum is albumen, fatty 

 matters, and a large proportion of a substance precipitable by 

 water. " The production of the chorion or shell-membrane 

 does not take place until the ovum has attained to nearly its 

 full size, and it appears to proceed, in part, from the consolida- 

 tion over the whole surface of one or more layers of an albu- 

 minous fluid secreted from the wall of the oviduct. The 

 observations of Herman Meyer have shown that a part of the 

 outer membrane is also 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. And many of the varieties in the appearance and 

 structure of the external covering, may probably depend on 

 the different modes of development of these cells. "* 



The embryo cell appears to be so directly connected with the 

 germinal vesicle, that at a certain period it is absorbed and 

 entirely disappears j or rather, ' ' the germinal yolk becomes 

 the nucleus of the future embryo, when a greater degree of 

 compactness is observed to take place in the yolk, and all that 

 remains of the germinal vesicle is one or more highly refract- 

 ing fat globules." In insects' eggs, as in those of the higher 

 animals, a clear space is seen between the surface of the yolk- 

 substance and the enclosing vitelline membrane. 



The shell is furnished with a lid, to facilitate, it is said, the 

 egress of the mature worm ; but since we find that the whole 

 integument offers little resistance to the strong and well 

 formed mandible of the creature, an operculum, or lid, seems 

 to be unnecessary for the pui'pose stated by continental 

 writers. The chorion, in many instances, is so very thin and 

 translucent, that even the changes taking place within can be 

 readily seen; indeed, I have often watched the young silk- 

 worm eat its way through the chorion, or egg shell, and this 

 its first trial of a formidable cutting instrument, well supplied 

 with muscular apparatus, it uses with admirable dexterity. 

 The head of the young caterpillar, according to the statement 

 of Meissner, lies towards the dot, or central opening in tho lid, 

 and which he has termed the micropyle,f from its resemblance 



* Dr. Allen Thomson. "Ovum, Cyclopedia of Anatomy and Physiology." 

 t Tlie term micropyle (a little gate) has heretofore only been used in connection 



wilh the vegetable kingdom ; it is used to denote the opening, or foramen, towards 



■which the radicle is phvays pointed. 



