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nious methods of concealing their eggs—for example, the burying-beetle deposits hers in the decaying carcasses of such animals as moles and mice, which they bury, if not already beneath the surface. It is stated by an observer of their operations, that in order to effect this object, several beetles unite their labours and remove the earth from beneath the dead body, which gradually sinks; they then proceed to cover it up, and frequently run backwards and forwards, apparently for the purpose of ramming down the earth. According to an eye-witness of the operations of these grave-diggers, four beetles were observed to inter in a very small space of earth, no fewer than twelve carcasses of various small animals. The object of all this care and solicitude, is not alone the security of their eggs, but to ensure an early supply of food for their young. Another example is afforded from among the Scarabæidæ, or larger kind of Dung-chaffers. The earth-borer (Geotrupes stercorarius), an insect whose “drowsy hum” falls so often on our ear, during a walk in the country in the stillness of an autumnal evening, digs round holes in the earth, often of considerable depth, then conveys a small quantity of dung to the bottom, in which the eggs are deposited. Each of these is placed in the centre of a small ball or pellet carefully prepared for this purpose; when dry enough, the pellet is transported, it may be to some distance from its place of preparation, to be buried in the hole dug for its reception, and what is somewhat remarkable, when it is unable to raise its load from the ground, it rolls it along, or pushes the pellet backwards with its hind legs. When the surface of the ground is irregular, the labour is proportionably increased, and not unfrequently the beetle is obliged to call in the assistance of its help-mate, before it can overcome the obstacles which impede it. According to some writers, the incessant and arduous labour which these beetles were observed to undergo, led the ancient Egyptians to regard them with a sort of sacred awe, and as symbolical of the labours of Osiris,
In form, colour, character, and beauty of design, the eggs of insects are more surprisingly varied than those of the feathered tribes ;* but our acquaintance with the composition of either exterior or interior is certainly not so complete as in the case of birds. The eggs of the animal series differ considerably in their external characteristics, nevertheless, all closely resemble each other, while yet a part of the ovarian ovum. At one period of their formation, all eggs consist of three nearly similar parts. First, The internal nucleated cell or germinal 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 membrane which encloses the whole, is the latest formed.
** The elaboration of structure and variety of forms in a large nunber of eggs, might be turned to a practical account, as many suggest patterns of great beauty and delicacy for art-designs.
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 consolidation over the whole surface of one or more layers of an albuminous 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; 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 refracting fat globules." In insects' eggs, as in those of the higher animals, a clear space is seen between the surface of the yolksubstance 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 purpose 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 silkworm 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 the lid, and which he has termed the micropyle,t from its resemblance to a small gate or opening, through which the worm is seen to emerge forth. From a number of observations made on silkworms' eggs, I have not been able to satisfy myself of the correctness of the particulars described by this observer, nor have I seen the young worm make its way out at this precise spot, but generally at a point much below it. Leuckart states this depression, the micropyle, becomes at a certain period converted into a funnel, which is directly connected with the mouth of the embryo, and serves to convey nourishment from without to it. I see no grounds for such a statement; because in the silkworm's egg, instead of a depression at this point, we have a nipple, and there can be no more necessity for leaving this funnel-shaped opening for the nourishment of the embryo catterpillar, than for that of the chick. There is no opening for such a purpose in the ovum of the bird ; indeed the vitelline membrame appears to form a perfectly closed sac to the yolk. In some eggs we appear to have an involuted portion of membrane, indicating simply where either the formative process of the outer membrane terminated, or the spermatozoa passed in to fecundate the yolk mass.*
* Dr. Allen Thomson. “Orum, Cyclopæ lia of Anatomy and Physiology."
+ The term micropyle (a little gate) has heretofore only been used in connection with the vegetable kingdom ; it is used to denote the opening, or foramen, towards which the radicle is always pointed.
The germinal vesicle is situated in the yolk mass, it is well marked, and of a very large size, in the egg of the bee, while the egg is yet in the ovasac. By preparing sections, after Dr. Hallifax's method, I find that the germinal vesicle of this insect is not situated immediately near, nor even below, the so-called micropyle, but more to the side of the yolk, represented in Fig. 17, and just in the position which the head of the embryo is found subsequently to occupy towards the end of the period of incubation. The germinal vesicle is well marked ; its macula is at first single, then becomes multiple.
The egg membrane, or shell as it is incorrectly called, of the moth and butterfly, is composed of three separate layers; an external slightly raised coat, tough and hard in its character; a middle one of united non-nucleated cells; and an inner one clear, dense, and homogeneous in structure, imparting a fine iridescent glaze to the surface, such as we see and admire in the old glass vessels exhumed from the ruins of Pompeii.
In the silkworm's egg the outer membrane consists of an inner reticulated membrane of non-nucleated cells, and an outer layer, the cells of which are arranged in an irregular circular manner, also non-nucleated, with a number of minute projecting interstitial hairs, or setæ. This layer has probably served, at some time, while the egg was yet a part of the ovarium ovum, as a vascular coat, and being no longer required, has become consolidated to form a part of the dense membranaceous covering.
* "As to the origin of the micropyle, it does not appear to proceed, as has been supposed by Meissner, from the mere deficiency of the epithelium cells in a certain space, and it is not dependent, either, on its pre-existence in the vitelline membrane, but, according to Leuckart, it is foued in the chorion before it appears in the vitelline membrane."-DR. ALLEN THOMSON.
It is in these several layers of the outer membrane that the micropyle apparatus is situated, and it should be noted that Meissner has described several varieties of the micropyle in the ova of insects belonging to the following genera, viz., muscue, tipula, pulex, lampyris, elater, teleophorus, odela, pyralida, tortrix, euprepia, liparis, pieris, panorpa, and in more than one species of several of these genera. The same author also observed and described in Musca vomitoria, a number of spermatic filaments entangled in the micropyle. Leuckart's observations, which are apparently more complete than those of Meissner, differ from his in some particulars; they extend over several hundreds of different kinds of insects' eggs, and he asserts that he succeeded in detecting the existence of the micropyle in not less than 200: he also gives detailed observations on this apparatus, and the structure of the membranes. I wil. lingly admit that such an extended series of observations fully entitles this author to great respect; and when we consider the minuteness of the eggs, and the difficulty of obtaining specimens in a suitable condition for investigations of the kind, we may appreciate the importance of the work, and the amount of labour required to bring such an investigation to a satisfactory conclusion. Leuckart positively asserts that, “in all instances in which the ova were ripe and favourable for examination, he was enabled to assure himself of the presence of the micropyle.” This supposed opening will be readily recognised in many of the drawings accompanying this paper, forming, as it does, a prominent spot at the pole of the egg, or in the middle of the circular radiants in the lid.
The outer surface of the egg shell of Coccus Persicce is covered by a series of minute rings, the ends of which somewhat overlap. These rings are believed by Sir John Lubbock to be identical in their character with the whitish substance which exudes through pores on the underside of the body; and it is more than probable that these layers of rings, and their arrangement, account for the beautiful prismatic hues which the egg presents under the microscope, when viewed as opaque objects. This substance, it appears, ultimately becomes a part of the intimate structure of the egg membranes. With regard to the greenish colour of the eggs of Phryganea, the same observer states " that it is due to the yolk-globules themselves.* In Coccus this is not the case, the yolk-globules are slightly yellow, and the green hue of the egg is owing to the green granules, which are only minute oil globules. When, however, the egg arrives at maturity, and the upper chamber has been removed by absorption, these green granules will be found to be replaced by dark green globules, regular in size, and about 1-8000th of an inch in diameter, and which appear to be in no way the same in the yolk of Phryganea egge.” Another curions fact has been noticed, which partially bears on the question of colour, the production of parasitic bodies within the eggs of some insects. In the Coccus, for instance, parasitic cells of a green colour occur, shaped like a string of sausages, in length about the 1-2000th of an inch, by about the 1-7000th of an inch in breadth. Of the formative process and composition of the colouring matter, not much is known. Valenciennes and Framy, after having bestowed attention on this part of the investigation, selecting the ova of Crustaceans for their purpose, arrived at the conclusion “that it is the same as that existing in the shell; which being green in the moist state, passes into a red colour when solidified.” By the aid of alcohol, etc., the colouring matter can be separated and collected in sufficient quantities for spectrum examination. The eggs of moths and butterflies, as I have before stated, present many varying tints of colour, and in speaking of this quality I do not restrict the term solely to the prismatic changes to which allusion has been made, and which are liable to constant mutations, according to the accident of the rays of light thrown upon them, but I more particularly refer to the several natural transitions of colour, the prevailing tints of which are yellow, white, grey, and a light brown. In some eggs the yellow, white, and grey are delicately blended; and these, when viewed with a magnifying power of about fifty diameters, and by the aid of a side-reflector (parabolic reflector), present many beautiful combinations, and the most delicate opalescent, or rather iridescent tints appear on others. The egg of the Mottledumber moth (Erannis defoliaria), Fig. 8, is in every particular very beautiful. It is ovoid, with regular hexagonal reticalations, each corner is studded with a white raised knob or button ; the space within the hexagon is finely punctuated, and the play of colours is exquisitely delicate. In this egg I have been unable to make out a micropyle. The Magpie moth (Abraxas grossularia), Fig. 1, is another example. The egg is ovoid, and somewhat resembles the former, but does not possess the raised stud or button, at the corner of each hexagon; its colour is very delicate, and silvery in tone. The membrane is so translucent that the movements of the young worm can be very well seen within. The egg of the Thorn moth (Ennomos erosaria), Fig. 5, is an elongated square form, one end of which is slightly tapered off, while the other is flattened; in this
* Pbil. Trans. 1859, p. 341.