MATURATION OF THE OVUM— FECUXDATIOX. 075 



germinal vesicle, — a phenomenon which occurs in all vertebrates, and 

 in a large proportion of, but probably not all invertebrate animals. This 

 change is iudependent of fecundation. The details of the process have 

 not been traced in mammals, but from various observations in birds 

 and batrachia, and more especially from the recent minute researches 

 of Oellacher in fishes, it follows that the disappearance of the vesicle 

 really depends on its extrusion from the substance of tlie yolk in which 

 it was imbedded, and is attended with the bursting or breaking down 

 of its delicate outer membrane ; so that when the vesicle is thrust out 

 on the surface of the yolk, and opened out, its fluid contents must be 

 effused in the space intervening between the vitelline membrane and 

 the surface of the yolk. The actual expulsion of the vesicle in the 

 trout's ovum is attributed by Oellacher to the contractions of the yolk 

 protoplasm, and the expulsion of the vesicle in this animal takes place 

 previous to the rupture of its membrane and dispersion of its contents. 

 (Archiv. f. Mikroscop. Anat. vol. viii, p. 24.) 



Fig. 490. — Mature Ovarian Ovum op the Pi^f. 490. 



GuiNEA-riG (from BischofF. ) ^so 



The zona pelhicida is hidden by the adherent ''A , » 



cells of the membrana granulosa, which have as- ^ ^ -> 



sumed a pediciilated form next its surface. The 

 finely granular yolk substance fills the cavity of '" 



the zona. The germinal vesicle has disappeared. 



The time at which the disappearance 

 takes place seems to be subject to some 

 variation. Most frequently it is close . 

 upon the time of the escape of the ^ 



ovum from the Graafian follicle ; but 



sometimes it is several hours later, and ^ -^ [ - -^ "^j" 



in other instances it seems to occur 

 previously ; and, indeed, in many cases 



preparatory changes in the position, form, and consistence of the vesicle 

 have been observed while the ovum was still within the ovary. 



As the mammiferous ovum leaves the ovary it has still adhering to 

 its outer surface one or two layers of the cells belonging to that part 

 of the tunica granulosa with which it was surrounded in the Graafian 

 follicle. These cells assume towards the period of maturation more or 

 less of a pediculated form (see fig. 490), but after one or two days they 

 gradually fall away from the surface of the zona, and leave tliat mem- 

 brane free in the Fallopian tube. 



Fecundation. — Should the ovum not be fecundated it is carried 

 down through the female passages by the ciliary action of the lining 

 membrane, and is lost by absorption or removal. But if seminal matter 

 is present in the tubes, and the ovum is subjected to its influence within 

 a due time, so that fecundation is effected, there immediately follows 

 the commencement of a series of changes in the yolk protoplasm, 

 which result in the formation in a determinate situation of a stratum 

 of organised cells constituting the laminar germ named the hiastoderm, 

 which is the seat of all subsequent processes of development in the 

 ovum. 



The encounter of the ovum with the seminal filaments or sperma- 

 tozoa generally takes place in the upper part of the Fallopian tube or 



X X 2 



