THE OVUM. 453 



attaining their present position, passed in succession through, the 

 several stages which are met with in passing from the surface 

 to the deeper parts of the ovary. 



The primitive ova are spherical cells, from 0'05 to O07 mm. 

 in diameter, with granular and rather ill-defined nuclei, and 

 devoid of nuclear membranes. Each primitive ovum is inclosed 

 in a follicle, consisting of a single layer of small cubical or 

 flattened epithelial cells. 



The permanent ova. About the time the egg follicles or 

 capsules commence to form around the primitive ova, these 

 latter undergo changes by which they become converted into 

 the permanent ova. Primitive ova occur in both sexes, and the 

 early stages in the development of the genital organs are the 

 same in both ; but the change to permanent ova occurs in the 

 female only, and marks the establishment of sexuality. 



The change, as in other animals, chiefly concerns the nucleus. 

 In the primitive ovum this is uniformly granular, with a rather 

 ill-defined outline ; in the permanent ovum it becomes converted 

 into a spherical vesicular body, of much larger size than before, 

 with a sharply defined double-contoured wall, fluid contents, 

 and a nuclear reticulum with one or more nucleolar enlarge- 

 ments at the nodes. 



Besides the changes in the nucleus, the whole egg increases 

 in size; its protoplasm, previously clear, becomes granular ; and 

 around the egg, between it and the follicle, a thin elastic invest- 

 ing membrane, the zona radiata, is formed. 



The Graafian follicle. Each ovum is surrounded at first by 

 a single layer of cells, derived, like the ovum itself, from the 

 germinal epithelium. These cells are at first flattened, but very 

 shortly become cubical or columnar in shape. Since they lie 

 between the ovum and the blood-vessels of the ovary, the nutrient 

 matter must pass through the follicular cells in order to reach 

 the ovum ; and it is probable that these cells do not merely 

 transmit the food, but play some part in elaborating it. 



A second layer of cells soon appears in each follicle, formed, 

 as in the rabbit, between the original layer and the ovum, and 

 probably by division of this originally single layer of cells 

 into two. Shortly afterwards, by further division, the follicle 



