112 LECTURE VI. 



whether these be present in the oviduct or not, the following changes 

 take place. The ovum becomes spheroidal and acquires a chorion ; 

 the vitelline granules become smaller ; the germinal vesicle disappears ; 

 and a number of large, transparent, oil-like globules appear, probably 

 the combined result of the condensation of the yolk granules and 

 the diffused contents of the germinal vesicle. The oil-like globules 

 approach the circumference of the ovum, and disappear ; the con- 

 densed yolk-mass assumes a spherical form in the centre of the egg ; 

 and the membrana vitelli is now recognised surrounding it, divided 

 by a clear fluid from the chorion. If impregnation have not pre- 

 viously taken place, the chorion is granulated, and such ova perish 

 without undergoing further developmental change. * 



When impregnation has taken place, the narrow oviduct is filled 

 by the flask-shaped spermatozoa, and the ovum, as it passes singly 

 into it from the ovarium, becomes surrounded by them. They indent 

 the previously well-defined surface of the tenacious granular yolk, 

 bury themselves in it, and partially break it up in the process. 

 Sometimes only one spermatozoon is thus seen embedded ; but more 

 commonly several spermatozoa penetrate the same ovum. The ger- 

 minal vesicle can be faintly recognised in some of these so penetrated 

 ova. As the ova proceed along the impregnated oviduct, the chorion 

 begins to be formed, as in the unimpregnated one. It appears at 

 first upon the smooth unperforated parts of the ovum, and afterwards 

 covers the ruptures themselves, and incloses the spermatozoa with 

 the yolk and its germinal vesicle. The inclosed spermatozoa now 

 lose their characteristic form, and swell into irregular masses having 

 a distinct outline, and, being highly refractive, give a mottled 

 appearance to the contents of the egg. They are then gradually 

 resolved into a transparent fluid, diffused, and effect changes, by 

 partial solution, on the yolk granules themselves, these changes pro- 

 ceeding from the surface towards the centre. The germinal vesicle 

 remains for a while in the centre of the ovum, more or less sur- 

 rounded by the undissolved yolk granules, whilst the egg acquires 

 one or two additional layers of the chorion secreted by the oviduct. 

 The germinal vesicle then ruptures or disappears ; but its nucleus, 

 with a now visible nucleolus, remains. The rupture of the germinal 

 vesicle is followed by a change in the character of the remaining yolk 

 granules, which become larger, less opaque, and float more loosely in 

 the fluid. The nucleus begins to swell out into a transparent cell — the 

 "primary germ-cell "f — of which the former nucleolus is now the 



* XCI. 



f First defined under that name, and so distinguished from its predecessor the 

 " germinal vesicle," in XXX. pp. 4, 5. 



