FERTILIZATION. 



11 



Fertilization. The ovum, after its expulsion from the Graafian follicle is 

 received upon the fimbriated end of the Fallopian tube. The fimbriae are covered by 

 a prolongation of the ciliated lining of the tube, and the action of the cilia serves to 

 propel the minute ovum into and along the tube towards the uterus. In this 

 passage it may, if impregnation have occurred, meet with the spermatozoa, one or 

 more of which may penetrate the zona pellucida, and fertilize the ovum. It is 

 possible in some instances for fertilization to occur on the fimbriated extremity of the 

 tube, or in the body of the uterus, but it is probable that in most cases it happens 

 in the tube itself. 



It is probable that normally only a single spermatozoon enters the vitellus. If it should 

 happen that two or more enter, normal development does not as a rule occur. Exceptions to 

 this rule have, however, been recorded. 



The changes in the ovum which accompany fertilization have, like those which 

 result in the formation of the polar globules, been studied most satisfactorily in the 

 transparent ova of echinoderms and in Ascaris. In the former (fig. 1 1) the spermatozoa 



Fig. 11. FERTILIZATION OP THE OVUM OP AN ECHINODERM. (Selenka.) 



s, spermatozoon ; m.pr, male pronucleus ; f.pr, female pronucleus. 



1. Accession of a spermatozoon to the periphery of the vitellus ; 2. Its penetration, and the radial 

 disposition of the vitelline granules ; 3. Transformation of the head of the spermatozoon into the male 

 pronucleus ; 4, 5. Blending of the male and female pronuclei. 



may be seen to penetrate the gelatinous investment which here takes the place of a 

 zona pellucida,. and the head, of one only as a rule, to imbed itself in the periphery 

 of the ovum, which becomes slightly protruded at the point of contact. According 

 to v. Beneden's account, the spermatozoon always enters in Ascaris at a particular 

 part of the ovum (polar disc), at which part there is an aperture in the vitellinc 

 membrane (micropyle). "When once it has passed into the ovum, this aperture 

 becomes closed, and the head of the spermatozoon rapidly increases in size, and 

 acquires the appearance of a nucleus which, in contra-distinction to the remains of 

 the germinal vesicle, or female pro-nucleus, is termed the male pro-nucleus. Soon 

 it leaves the periphery, and passes towards the centre of the ovum in the direction of 

 the female pro-nucleus. In its passage through the protoplasm it appears to exer- 

 cise a peculiar attraction upon the granules in that substance, for these become 

 arranged in its vicinity in radiating lines. The tail of the spermatozoon 

 has in the meantime disappeared, whether by being cast off or by blending with the 

 protoplasm of the ovum has not certainly been made out. As the male pro-nucleus 

 approaches the female pro-nucleus, the latter moves somewhat to meet it, and pre- 

 sently the two pro-nuclei come into contact and together form a new nucleus, com- 



