FERTILISATION OF THE OVUM 379 



§ 74. Fertilisation of the Ovum. 



The ovum, when expelled from the Graafian foUicle of the ovary, is caught by the fimbriated extremity of the 

 Fallopian tube, and transmitted by cihary action along the tube in the direction of the uterus. If coitus has 

 taken place it may, and generally does, encounter the male element (spermatozoon) in the tube. Less frequently, 

 impregnation occurs in the uterus itself. Impregnation may, as already indicated, occur outside both the tube and 

 the uterus, in which case the impregnated ovum may fall into the abdominal cavity and produce extra-uterine gesta- 

 tion. As a rule only one spermatozoon is required for fertilisation. The changes incidental to fertilisation are best 

 seen in transparent ova such as those of echinoderms and ascaris. In echinoderms one spermatozoon generally 

 may be seen to enter the gelatinous envelope which takes the place of the zona pellucida, and thrusts its head into 

 the outer surface of the ovum, which rises up to meet it (Plate Ixxxvi., 0, P, Q, R, S, page 383). 



In Ascaris, according to E. v. Beneden, a special aperture in the vitelline membrane (micropyle) is provided for 

 the entrance of the spermatozoon, this always entering at the same part of the ovum (polar disc). The aperture in 

 question is closed soon after the male element has passed in. The relations between the ovum and spermatozoon 

 are in E. v. Beneden's opinion well defined and exact. They are obviously pre-arranged and the outcome of design. 



When the head of the spermatozoon has penetrated the ovum it quickly increases in size, and assumes the 

 appearance of a nucleus. It forms the male pronucleus, and is to be distinguished from the female pronucleus formed 

 by the remains of the germinal vesicle. The male pronucleus travels towards the centre of the ovum in the direction 

 of the female pronucleus, and in so doing causes the molecular and granular contents of the ovum to arrange them- 

 selves in well-marked and very remarkable radiating and concentric lines. The male pronucleus is evidently a 

 directive force. When it reaches the vicinity of the female pronucleus the latter moves towards it as if by pre- 

 concerted arrangement. The male pronucleus at this juncture throws off its vibratile tail, it being no longer 

 required. The male and female pronuclei when they come together both take part in the stellate-concentric arrange- 

 ment of the contents of the ovum above referred to. They form by their union a new nucleus, capable of forming 

 new cells by division. 



Fertilisation is to be regarded as " the conjunction of part of the nucleoplasm of a sperm cell (protozoon) with 

 part of the nucleoplasm of a germ cell (ovum) ; the result being the production of a complete nucleus endowed with 

 active properties of division and reproduction." 



The peculiar structures and properties possessed by the nucleus as seen in the cell of the larva of the salamander 

 are illustrated at Plate Ixxxvii., A to Q inclusive. In this case the contents of the nucleus when resting are arranged 

 in a network. When active and the nucleus is dividing they pass through a series of remarkable changes. Thus 

 in the first stage of division the chromoplasm is transformed into a skein of closely contorted filaments ; in the 

 second stage, the filaments become larger and looser ; in the third stage, the filaments become still larger, and 

 present a looped appearance ; in the fourth stage, they are arranged rosette-fashion with a central clear space ; in 

 the fifth stage, the filaments converge and present a stellate appearance. The filaments subsequently split up 

 longitudinally, and divide into two sets (metakinesis) to form daughter nuclei. The daughter nuclei ultimately 

 separate, and repeat in their life histories the history of the parent nucleus just given. It will be seen that the 

 nucleus is by no means the simple structure it was once supposed to be. On the contrary, it is highly complex ; 

 its activity being correspondingly great. 



It would be impossible to exaggerate the importance of atoms, molecules, ova, cells, nuclei, and other rudi- 

 mentary structures cormected with reproduction. As these structures exhibit from the first, and on the very 

 threshold of life, globular, concentric, curved, spiral, radiating, branched, and segmented arrangements, it behoves 

 us to give more than passing attention to living matter in the young and adult states, and to dead matter as a 

 constituent of the inorganic kingdom. 



Much and general misunderstanding exists as to the relations between dead and Uving matter in the universe. 

 It is, for example, not generally recognised that the inorganic and organic kingdoms are made for each other and 

 obey similar laws ; that they are co-ordinated and mutually interdependent ; that they have much in common, 

 and are not opposed to or destructive of each other ; that impassable limits are set to both ; that hfe is adminis- 

 tered and kept going by the outside world ; that while individuals die off in time and space, the types of the race 

 continue to live and to persist from generation to generation ; the stock of Ufe being, as it were, transmitted 

 unimpaired by innumerable ancestors, which have a continuous existence, and only reproduce their own kind. 



In the whole range of biology there is no more outstanding feature than is furnished by plant and animal types 

 and by the stability and persistency of these types. Many millions of plants and animals have inhabited and con- 

 tinue to inhabit the earth, but (and this is the pecuUarity) they have not become mixed up or lost their identity to 

 any appreciable extent. This is an extraordinary circumstance, and cannot be too deeply pondered. 



