OVUM. 



[137] 



doubt the correctness of the view hitherto 

 generally adopted as to the entire disappear- 

 ance of the germinal vesicle in that instance, 

 and holds it as probable that a part of it at 

 least remains in connection with the forma- 

 tion of the embryonic cell. These statements 

 are sufficient to show that the phenomena of 

 the dehiscence of the germinal vesicle and 

 its relation to the subsequent changes in the 

 ovum induced by fecundation are as yet very 

 imperfectly understood, and that the discovery 

 has still to be made of the link in the chain 

 of connection between the last stage of 

 existence of the ovigerm, and the first origin 

 of the nucleus round which the subsequent 

 organising process of segmentation begins. 

 But that some such connection exists, all 

 who have made a study of this part of the his- 

 tory of the ovum are inclined to believe, 



5. Relation of the ovum to fecundation by 

 the male sperm. 



The act of fecundation is necessary for the 

 perfection of all true ova. In the production 

 of gemma? or buds, in the multiplication of 

 nonsexual individuals, and in the various 

 examples of Metagenesis previously referred 

 to, the germs from which the new products 

 arise may be nucleated cells or groups of 

 these, and may without doubt be the descend- 

 ants of the original cell-germs of ova; but 

 for their development into the new beings 

 produced from them, no combination, so* far 

 as is yet known, with the product of cells of 

 a different kind, as in fecundation, is necessary. 

 It is otherwise with all true ova. Their 

 germs may be the descendants through the 

 ovary of an original cell-germ, from which 

 the animal bearing the ovary was produced ; 

 but for the generation of an ovum the ovigerm 

 must be subjected to the influence of the 

 sperm, and for its development there is re- 

 quired a new process of organisation, inaugu- 

 rated by segmentation, which is the invariable 

 consequence of fecundation, and is the first 

 obvious change in a fecundated ovum leading 

 to embryonic formation. 



The developed form of the spermatic sub- 

 stance * is in by far the greater number of ani- 

 mals that of minute ovoid or rounded particles of 

 various form, with each of which is connected 

 a long and extremely delicate filament, which 

 moves with vivacity in a vibrating or oscil- 

 latory manner when immersed in water and 

 various bland animal solutions. There are 

 other less common forms of spermatozoa, 

 such as those of Crustacea and Nematoidea, 

 which have not the filamentous appendage, 

 and are motionless. The vibratory motion 

 of filamentous spermatozoa bears some resem- 

 blance to that of some kinds of fine cilia, and 

 is the most apparent indication of the active 

 state of their vitality.f 



It is now ascertained beyond doubt that in 

 a number of animals the spermatozoa come 

 into direct contact with the yolk substance 



* See the article SEMES. 



t See especially the recent researches of Kolliker 

 on the Sperm in Zeitsch. fur, Wissensch. Zool. 

 v ol. vii. 



and embryogerm, or with the internal con- 

 tents of the ovum. The actual entrance of 

 the spermatozoa into the ovum has been 

 observed in Mammalia, Batrachia, Osseous 

 Fishes, Insects, Nematoid Worms, some 

 Mollusca, and Echinodermata ; and there 

 have been ascertained circumstances regarding 

 the ova of other animals which warrant the 

 inference that the spermatozoa enter the 

 ovum in many more than those in which the 

 phenomenon has already been actually ob- 

 served. After long continued doubt and 

 much discussion of this point, physiologists 

 are therefore now generally agreed that in 

 all instances a direct action of the sperma- 

 tozoa in substance on the contents of the 

 egg is necessary to fecundation. The manner 

 of access of the spermatozoa to the interior 

 of the ovum is probably various in different 

 animals. In a few, as Trematode and Cestoid 

 Entozoa, the sperm is mixed with the contents 

 of the ovum, viz., the germinal vesicle and 

 yolk, at the time when these are brought 

 together from the separate organs in which 

 they are formed : in some, as the Nematoid 

 Worms, and probably also in some other 

 animals, the sperm comes in contact with the 

 ovum previous to the formation of an en- 

 veloping membrane ; in a third set it seems 

 probable that, as in Lumbricus, and perhaps 

 in some Mollusca and Hirudinea, the vitelline 

 membrane which had existed at an earlier 

 period is dissolved or removed previous to 

 fecundation, and the ovum or yolk substance 

 and germ are thus left directly exposed to 

 the action of the spermatozoa, which in Lum- 

 bricus have been observed in great numbers 

 penetrating the substance of the yolk. 



In the majority of animals, however, the 

 sperm only reaches the ovum at a later stage 

 of its formation, when it is already covered by 

 the vitelline membrane or other envelopes, 

 and through these coverings, therefore, the 

 spermatozoa must pass to gain access to the 

 yolk and germ. In a certain number of ani- 

 mals the vitelline or enveloping membrane 

 appears to be quite entire and closed on all 

 sides, so that, as in Mammalia, in which 

 Martin Barry was the first in 1843 to perceive 

 with certainty the entrance of the spermatozoa 

 into the ovum, these bodies must in some 

 way, not yet fully known, pass through the 

 consistent wall of the enclosing membrane ; 

 but in other animals, as first discovered by 

 J. Mtiller, a special aperture or perforation of 

 the egg- covering exists, apparently destined 

 to allow of the more rapid entrance of the 

 spermatic bodies. This micropyk apparatus, 

 sometimes consisting of one, and at others of 

 a number of apertures, has now been observed 

 in several Echinodermata, in Acephalous 

 Mollusca, in all Insects, and in Osseous Fishes ; 

 and it is more than probable that it exists in 

 a considerable number of other animals in 

 which it has not yet been detected. But 

 still, making due allowance for the probable 

 extension of discovery in this direction, the 

 care and accuracy with which the micropyle 

 apparatus has since its first discovery been 

 sought for without success in Mammalia and 



