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§ 74. Fertilisation of the Ovum.<br />
FERTILISATION OF THE OVUM 379<br />
The ovum, when expelled from the Graafian foUicle of the ovary, is caught by the fimbriated extremity of the<br />
Fallopian tube, and transmitted by cihary action along the tube in the direction of the uterus. If coitus has<br />
taken place it may, and generally does, encounter the male element (spermatozoon) in the tube. Less frequently,<br />
impregnation occurs in the uterus itself. Impregnation may, as already indicated, occur outside both the tube and<br />
the uterus, in which case the impregnated ovum may fall into the abdominal cavity and produce extra-uterine gesta-<br />
tion. As a rule only one spermatozoon is required for fertilisation. The changes incidental to fertilisation are best<br />
seen in transparent ova such as those of echinoderms and ascaris. In echinoderms one spermatozoon generally<br />
may be seen to enter the gelatinous envelope which takes the place of the zona pellucida, and thrusts its head into<br />
the outer surface of the ovum, which rises up to meet it (Plate Ixxxvi., 0, P, Q, R, S, page 383).<br />
In Ascaris, according to E. v. Beneden, a special aperture in the vitelline membrane (micropyle) is provided for<br />
the entrance of the spermatozoon, this always entering at the same part of the ovum (polar disc). The aperture in<br />
question is closed soon after the male element has passed in. The relations between the ovum and spermatozoon<br />
are in E. v. Beneden's opinion well defined and exact. They are obviously pre-arranged and the outcome of design.<br />
When the head of the spermatozoon has penetrated the ovum it quickly increases in size, and assumes the<br />
appearance of a nucleus. It forms the male pronucleus, and is to be distinguished from the female pronucleus formed<br />
by the remains of the germinal vesicle. The male pronucleus travels towards the centre of the ovum in the direction<br />
of the female pronucleus, and in so doing causes the molecular and granular contents of the ovum to arrange them-<br />
selves in well-marked and very remarkable radiating and concentric lines. The male pronucleus is evidently a<br />
directive force. When it reaches the vicinity of the female pronucleus the latter moves towards it as if by pre-<br />
concerted arrangement. The male pronucleus at this juncture throws off its vibratile tail, it being no longer<br />
required. The male and female pronuclei when they come together both take part in the stellate-concentric arrangement<br />
of the contents of the ovum above referred to. They form by their union a new nucleus, capable of forming<br />
new cells by division.<br />
Fertilisation is to be regarded as " the conjunction of part of the nucleoplasm of a sperm cell (protozoon) with<br />
part of the nucleoplasm of a germ cell (ovum) ; the result being the production of a complete nucleus endowed with<br />
active properties of division and reproduction."<br />
The peculiar structures and properties possessed by the nucleus as seen in the cell of the larva of the salamander<br />
are illustrated at Plate Ixxxvii., A to Q inclusive. In this case the contents of the nucleus when resting are arranged<br />
in a network. When active and the nucleus is dividing they pass through a series of remarkable changes. Thus<br />
in the first stage of division the chromoplasm is transformed into a skein of closely contorted filaments ; in the<br />
second stage, the filaments become larger and looser ; in the third stage, the filaments become still larger, and<br />
present a looped appearance ; in the fourth stage, they are arranged rosette-fashion with a central clear space ; in<br />
the fifth stage, the filaments converge and present a stellate appearance. The filaments subsequently split up<br />
longitudinally, and divide into two sets (metakinesis) to form daughter nuclei. The daughter nuclei ultimately<br />
separate, and repeat in their life histories the history of the parent nucleus just given. It will be seen that the<br />
nucleus is by no means the simple structure it was once supposed to be. On the contrary, it is highly complex ;<br />
its activity being correspondingly great.<br />
It would be impossible to exaggerate the importance of atoms, molecules, ova, cells, nuclei, and other rudi-<br />
mentary structures cormected with reproduction. As these structures exhibit from the first, and on the very<br />
threshold of life, globular, concentric, curved, spiral, radiating, branched, and segmented arrangements, it behoves<br />
us to give more than passing attention to living matter in the young and adult states, and to dead matter as a<br />
constituent of the inorganic kingdom.<br />
Much and general misunderstanding exists as to the relations between dead and Uving matter in the universe.<br />
It is, for example, not generally recognised that the inorganic and organic kingdoms are made for each other and<br />
obey similar laws ; that they are co-ordinated and mutually interdependent ; that they have much in common,<br />
and are not opposed to or destructive of each other ; that impassable limits are set to both ; that hfe is adminis-<br />
tered and kept going by the outside world ; that while individuals die off in time and space, the types of the race<br />
continue to live and to persist from generation to generation ; the stock of Ufe being, as it were, transmitted<br />
unimpaired by innumerable ancestors, which have a continuous existence, and only reproduce their own kind.<br />
In the whole range of biology there is no more outstanding feature than is furnished by plant and animal types<br />
and by the stability and persistency of these types. Many millions of plants and animals have inhabited and con-<br />
tinue to inhabit the earth, but (and this is the pecuUarity) they have not become mixed up or lost their identity to<br />
any appreciable extent. This is an extraordinary circumstance, and cannot be too deeply pondered.