[no] 



OVUM. 



cussata, according to Lej'dig*, the ova are ar- 

 ranged in aggregated pediculated groups, from 

 which it seems probable that they are originally 

 produced in numbers by the multiplication or 

 division of multiple germs, somewhat in the 

 same manner as will afterwards be stated to 

 have been observed by Meissner among the 

 Gordian Nematoid Worms. An albuminous 

 layer is afterwards formed externally, and may 

 be instrumental at last, from its increasing 

 thickness, in separating the ovum from its pe- 

 diculated attachment to the ovary. There 

 seems therefore to be little doubt that in these 

 Mollusca, and in a certain number of other 

 Invertebrate animals in which the micropyle 

 has been observed, this apparatus is produced 

 by the remains of an original or early ovarian 

 pedicle. In the Unio and Anodonta it is 

 certainly not formed by the peculiar process 

 of development from within the ovum, which 

 has been elaborately described by Keber.f 

 It will afterwards be shown, however, as in- 

 deed may be concluded from what has already 

 been stated in regard to osseous fishes, that 

 in other animals the micropyle may arise in 

 other modes and without the early existence 

 of the pedicle now described. 



When the ova are detached by the rupture 

 of the pedicle in the Acephala, they lie, in 

 different stages of advancement, but all pro- 

 vided with the micropyle, in the general ovarian 

 cavity. The coverings of the Acephalous 

 ovum appear to be composed at least of 

 two layers, of which the inner may per- 

 haps be looked upon as the vitelline mem- 

 brane, the outer as a chorion; but sufficient 

 data have not yet been furnished to determine 

 the homological rank of these membranes. 

 The early connection, in a pediculated form, 

 with the ovarian stroma might point to a 

 different view of their nature. Leydig states 

 that while in Unio and Anodonta the albu- 

 men is deposited within the membranes, in 

 Venus it is added externally. The micropyle 

 appears to be closed previous to the com- 

 mencement of embryonic development. 



Arthropoda. The ova of Articulate animals 

 might with most propriety be classed with 

 the large-yolked group, at least as regards the 

 ova of Insecta, Arachnida, and the higher 

 Crustacea. In addition to the germinal vesicle 

 and finely granular yolk-substance, they all 

 contain a large proportion of clear or oil glo- 

 bules of considerable size ; and the process of 

 segmentation is generally limited to a small 

 portion of the yolk surface. The ova of 

 these three classes present, however, many 

 subordinate differences in their structure and 

 mode of production, which renders it neces- 

 sary to give a short separate account of them 

 in this place. 



Insecta. The ova of insects are more 

 especially distinguished by the extraordinary 

 varieties of their external form and appearance. 

 These varieties affect, however, principally, 

 or depend upon modifications of the external 



* Muller'sArchiv. 1854, p. 320. 

 f Hessling and BischoiF, loc. cit. 



covering, chorion or shell-membrane, as it has 

 been called. They differ also from those of 

 most other animals in a frequent departure 

 from the regular symmetrical form. Some 

 are nearly hemispherical, others more oval ; 

 many are somewhat bent in an antero-posterior 

 direction* ; many present the most curious 

 elevations and irregularities on their external 

 surface reticulated ridges or fringes, and de- 

 pressions, tubercles, hairs or spines, or other 

 long processes, sometimes single, at other 

 times in numbers. These modifications 

 of the external coverings of the eggs of 

 Insects appear to have reference chiefly to 

 the protection of the ova from the effects of 

 external injury, and to serve various mechani- 

 cal purposes connected with their deposition 

 and attachment ; but they are not, in most at 

 least, attended with any important varieties in 

 the internal structure, which, on the whole, 

 presents considerable uniformity throughout 

 the whole class. The ova of all insects, we 

 shall afterwards see, are provided with one or 

 more apertures corresponding to the micro- 

 pyle. , 



All recent observers agree that, in the ova 

 of Insects, in addition to the external shell- 

 covering, there is a delicate transparent vitel- 

 line membrane. The germinal vesicle is of pro- 

 portionately large size. Its macula is at first 

 single ; but in the course of the growth of the 

 ovum it becomes multiple, or diffused as a finely 

 granular or molecular substance throughout 

 the vesicle. -j- The germinal vesicle is situated 

 in a vitelline or germinal area composed of fine 

 granules, in which without doubt the limited 

 process of segmentation afterwards takes place ; 

 but fuller observations are still much required 

 in regard to the segmentation of the yolk in 

 insects, which has as yet been very rarely 

 seen. The germinal vesicle appears to be 

 burst and diffused at a comparatively early 

 stage of the growth of the egg. 



The external membrane consists in general 

 of more than one layer of substance. The 

 outer and inner are described as being gene- 

 rally more clear, dense, and homogeneous; 

 the middle one, in some insects at least, pre- 

 senting greater varieties of structure, and not 

 unfrequently being composed of united nu- 

 cleated cells. It is in these several layers of 

 the outer membrane that the micropyle ap- 

 paratus, recently discovered, is situated. The 

 existence of a micropyle in the ova of Insects 

 was first published by Meissner, in September, 

 18541 ; but the discovery appears to have 

 been made simultaneously by Leuckart, who 

 has given a most interesting and elaborate 

 description of this apparatus, and of the 

 minute structure of the membranes, in a great 

 variety of insects, in a memoir recently pub- 

 lished by him.$ 



Meissner described several varieties of the 



* This has reference to the position they occupy 

 during their formation in the passages of the female 

 parent. 



f See R. Wagner's Prodromus Hist. Generat. 



J Zeitsch. fur Wissen. Zool. vol. v., p. 272. 

 Memoir on the Micropyle and Minute Structure, 



