xin PHYLUM CHORDATA 569 



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 distinguishable among the protoplasm. 



As the ovum approaches maturity the fluid liquor folliculi 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 wall 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 (not always present), 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 

 corpus 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. 1227) 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 

 meroblastic type, 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 Monotremes, segmentation is complete 

 or holoblastic, 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 approximately 

 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 embryonal 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 liquid, a cavity, 

 which is formed in the interior of the sphere, increases rapidly in 

 size. The stage now reached is called the blastodermic vesicle. 

 During the growth in size of the internal cavity the central mass 

 of cells remains in contact with one side only of the outer layer, 

 where it spreads out as a stratum several cells deep. From it are 



