xiri VIIVLUM CHORDATA 587 



cells, iscpamtcs an outer layer uf Lhu tollicle cells — the mcinhraivt 

 gramdoM (memh.) — from the mass — cunmlits iiroligcrus {disc.) — sur- 

 rounding the ovum, except on one side where they coalesce. 

 A basement membrane is formed externally to the follicle cells, 

 and the stroma around this becomes vascular, and forms a two- 

 layered investment for the follicle. The cells immediately 

 surrounding the ovum become arranged as a definite layer of 

 cylindrical cells — the corona radiata. A thick membrane — the 

 zona radiata — perforated by numerous radially arranged pores, into 

 which project processes from the cells of the corona, invests the 

 ovum ; and in many, if not in all, there is beneath this a delicate 

 vitelline membrane. As the ovum increases in size, granules of 

 yolk become distinguisliable among the protoplasm. 



As the ovum approaches maturity the fluid — li(];uor follicidi — in 

 the cavity of the follicle increases in quantity, so that the follicle 

 becomes greatly distended. The follicle has meanwhile approached 

 the surface of the ovary, on which it comes to project as a rounded 

 prominence. Eventually the middle region of the projecting part 

 of the Avail of the follicle thins out and ruptures, setting free the 

 ovum, which passes into the Fallopian tube. On the way along 

 the Fallopian tube impregnation takes place, and, after becoming 

 enclosed in an envelope of albumen, the ovum passes onwards to 

 the uterus, there to undergo its development. In the place of the 

 discharged ovum there is left a space which becomes filled with 

 connective-tissue to form a body known as the corjjus luteum. If 

 the ovum should become fertilised and proceed to develop in the 

 uterus, the corpus luteum increases in size and persists for a 

 considerable time : if no development takes place it disappears 

 comparatively quickly. 



With the absence of food-yolk are connected most of the differ- 

 ences observable between the early stages of the development of a 

 higher Mammal (Fig. 1215) and the corresponding stages in the 

 development of a Reptile or Bird. One of the most striking of these 

 is in the mode of segmentation. In the case of the large ovum of the 

 Bird, as we have seen, the segmentation is of the incomplete or 

 merohlastic tj^pe, being confined to a small disc of protoplasm — the 

 germinal disc — on one side of the ovum. In the Mammals, on the 

 other hand, except in the Moaotremes, segmentation is- complete 

 or liolohlastic, the entire ovum taking part in the process of seg- 

 mentation. The segmentation is nearly or quite regular, the 

 cells into which the ovum divides being of equal, or approxi- 

 mately equal, size. The result, in the Eutheria, is the formation 

 of a sphere of cells, which soon become distinguishable into an 

 outer layer and a central mass, the inner cell-mass or emhryoncd 

 knot. In the Marsupials, so far as known, the stage of a solid 

 cellular sphere or morula does not occur, a central cavity being 

 present from the outset. In the Eutheria, by imbibition of 



