[112] 



OVUM. 



Fig. 77*. 



Oeum and Slicropyle of Dipterous Insects. (From 

 Leuckart.) 



A. Ovum of Melophagus ovinus (Muscida). 1. 

 The entire ovum, presenting at its upper part the 

 adherent mass of spermatozoa close to the micro- 

 pyle. 2. This upper part more highly magnified, 

 showing a section of the micropyle, above which 

 the point of the conical mass of spermatozoa glued 

 together by an albuminous substance is inserted, 

 while externally the filaments float free. 3. The 

 micropyle apertures seen directly from above. 



B. Side view of the upper part of the ovum of 

 another insect of the same order, showing a single 

 micropyle aperture and the dotted structure of the 

 chorion. 



small mass, formed of the spermatozoa which 

 have met with the ovum in its descent through 

 the female passage, comes to be lodged in the 

 depression of the micropyle, and is fixed in 

 that situation by a lid or covering of albu- 

 minous matter. It is somewhat remarkable 

 that the greater part of this mass remains for 

 a long time apparently without any change, 

 even when embryonic development has ad- 

 vanced to a considerable extent ; but he as- 

 certained that a few of the spermatozoa be- 

 longing to the mass, usually not more than 

 three or four, really enter the ovum and effect 

 the change of fecundation. We are, however, 

 as yet at a loss to conjecture what farther 

 purpose may be served by the mass of per- 

 sistent spermatozoa near the micropyle. 

 Leuckart has also made the novel and 

 interesting observation, that the depression 

 and aperture of the micropyle become at 

 a later period converted into a deeper funnel, 

 which is connected directly with the mouth 

 of the embryo, and undoubtedly serves, ac- 

 cording to this author, to convey nourishment 

 from without to the embryo. The head of 

 the embryo lies, according to Leuckart and 



other observers*, in all instances, at that end 

 or pole of the ovum which is uppermost in 

 the oviduct, as may be most easily observed 

 in ova of the cylindrical form, such as those 

 of the common house-fly ; but according to 

 Leuckart, the micropyle is not, as Meissner 

 had stated, always at that end, being some- 

 times at one, sometimes at the other, and oc- 

 casionally at both poles. The provision for 

 the escape of the embryo, however, is usually 

 at the upper or anterior pole, while the lower 

 or hinder pole more generally serves to fix 

 the ovum, as it is often pediculated or other- 

 wise modified in its form in connection with 

 this purpose. 



In some Insects, as is shown in the accom- 

 panying figure of the ovum in Pulex irritans, 

 the micropyle consists of a number of foramina 

 nearly of uniform size. 



Fig. 78*. 



Ovum of Pulex irritans. (From Leuckart.) 



A. Entire ovum, magnified, showing the micro- 

 pyle apparatus with a number of foramina at both 

 poles. 



B. Portion of the chorion with the micropyle 

 foramina, more highly magnified. 



In a previous part of this article allusion 

 has already been made to the great facility 

 with which the development of the ova of in- 

 sects may be traced, in their successive stages, 

 as they lie in different parts of the tubular 

 ovaries and oviducts. According to the In- 

 teresting observations of R. Wagner -{-, the 

 upper end of the fine ovarian tubes are filled 

 with a number of germinal vesicles. Wag- 

 ner supposed indeed that these were at first 

 nucleoli or germinal maculae, and that a vesicle 

 was developed round each macula ; but Leuck- 

 artj and Steins) were never able to detect the 

 germinal vesicles before they already possessed 

 the macula. The primitive yolk arises as in 

 most other animals first, by the collection of 

 a clear substance immediately round the 

 germinal vesicle, and by the subsequent de- 

 posit in this matrix of the fine granules of 

 the vitelline substance ; later still the deli- 

 cate vitelline membrane is formed, perhaps 

 by the consolidation of a film of the primitive 

 yolk-substance. 



As the ova attain a larger size, each one being 

 situated in the lower part of its compartment 



* See Kblliker, de prima Insector. Genesi, 4to. 

 Turici, 1842. 



f Prodromus, Hist. Gener. p. 9., and Beitriige zur 

 Entwickel., &c. p. 42. See Jig. 40. Append, of 

 CYCLOP. ANAT. AND PHYSIOL. 



i Zeugung, p. 803. 



Vergleich. Anat. und Physiol. der Insecten, 

 Berlin, 1847. 



