[106] 



OVUM. 



which subdivide them; and as these cross folds 

 are formed the longitudinal ones beome gradu- 

 ally shallower. The surface of the egg then 

 presents the reticulated appearance which is 

 shown in fig. 70.* On making a section 

 through such an egg, hardened^ in alcohol or 

 any other suitable reagent, it is easy to per- 

 ceive that the ovicapsule takes no part in the 

 inflections, but that they consist entirely in the 

 grooving of the yolk, and the corresponding 

 bending into the grooves of the vitelline mem- 

 brane. This state is maintained till the ovum 

 is approaching maturity, when the depth of 

 the grooves or folds speedily diminishes; and 

 these come at last to be completely effaced in 

 those ova which have left the ovicapsule. 

 In Loligo, it is stated by Kolliker, there are 

 only the longitudinal folds. No satisfactory 

 opinion has been offered as to the cause of 

 this peculiar structure. 



Fig. 70 *. 



Ova of the Sepia. (.From Kolliker.') 



A. Three ovarian ova of the Sepia in somewhat 

 different stages of advancement attached by their 

 pedicles to the ovary, and represented several times 

 magnified. They all show the reticulated mark- 

 ings on the surface produced by the folding in of 

 the vitelline membrane ; g, the place of the germi- 

 nal vesicle and possibly also of a micropyle at the 

 small pole of the egg, in which segmentation after- 

 wards occurs. 



B. Direct view of this germinal pole of one of 

 the ova, showing the absence of the folds towards 

 the centre in which the germinal vesicle is situated. 



c. Cross section of one of the ova, showing at o 

 the unfolded or smooth ovarian capsule, and at v m 

 the folded vitelline membrane. 



Towards the narrow pole of the ovum, the 

 folds now described become less marked ; and 

 they are entirely absent just at the pole itself, 

 so that the germinal vesicle may be seen in the 



smooth space which is left between them. At 

 this place I had some expectation to find an 

 aperture of the nature of a micropyle ; and I 

 accordingly sought for it in some specimens of 

 the ovarian ova of Sepia which I had pre- 

 served in alcohol, but without success, per- 

 haps on account of the opacity produced in 

 the membranes by the alcohol, and the adhe- 

 sion of the yolk substance to them. Professor 

 Kolliker has since informed me that he believes 

 the micropyle to exist in these ova, which I 

 think extremely probable. 



The germinal vesicle, according to this ob- 

 server, remains entire and visible till the ova 

 are mature, as may be seen by the examination 

 of specimens hardened in alcohol. It dis- 

 appears just about the time of the ova leaving 

 the ovarian capsule ; but in several instances 

 he found it still remaining in ova that were 

 already free. It was always gone in those ova 

 which had regained the smoothness of their 

 exterior. 



The yolk-substance of the mature ova con- 

 sists entirely, excepting immediately at the 

 seat of the germinal disc, of corpuscles some- 

 what similar to the vitelline tablets of the Frog's 

 egg. At an earlier period there are heaps of 

 fine granules of the same size, from which the 

 corpuscles are therefore probably formed. In 

 the earliest stage the vitelline substance is 

 entirely composed of fine molecules the pri- 

 mitive yolk which appear to be formed in 

 the same manner as in other animals. 



The Cephalopoda furnish a remarkable ex- 

 ample among the Invertebrata of a very limited 

 or partial segmentation. This process, upon 

 the detailed description of which I will not 

 enter here, usually commences in a spot either 

 in or more frequently near to the germinal 

 pole, by the formation of the primitive groove 

 which extends across the disc. The forma- 

 tion of the second groove, which crosses the 

 first, the production of other radiating 

 grooves, the separation of annular sets of 

 segments from the periphery, and the suc- 

 cessive steps of the process which follow, are 

 probably determined by the same circum- 

 stances which have been referred to as re- 

 lated to this phenomenon in the cicatricula of 

 the Bird's egg. 



Gasteropoda. In the greater number, if not 

 almost all, of the remaining Mollusca, the 

 ova differ greatly from those of the Cephalo- 

 poda, and approach more nearly to those I 

 have classed under the groups possessing the 

 small or middle sized yolk, which is princi- 

 pally or entirely formative, and which under- 

 goes a more or less complete segmentation.* In 

 the Pteropoda and in those of gasteropodous 

 Mollusca, which have the male and fe- 

 male organs in the same individual, there is 

 a remarkable combination of the ovary and 

 testis in a single hermaphrodite organ, usually 



* The genus Sagitta, among the Pteropoda, is, 

 however, probably an exception to this statement, 

 as in it, according to Darwin, the embr3 r onal part 

 of the yolk is distinct from the rest, or rather covers 

 it like a ring. 



