404 
Avgas persicus 
ovarian epithelium the ova (o. o'.) are derived and in a young female 
they may be recognised in all stages of development, scattered among 
the smaller unaltered cells which are always present in large numbers. 
As the ova increase in size, they approach the basal membrane of 
the ovarian epithelium and, simultaneously, a proliferation of the 
adjacent unaltered epithelial cells takes place with the result that a 
pedicel (p. ov .) is formed, upon the extremity of which the rapidly 
growing ovum is pushed out of the epithelium, carrying the basal 
membrane before it, as a closely investing ovarian follicle (f ov.). In 
this manner the ovum finally comes to rest on the external surface of 
the ovarian epithelium, with which it remains connected by the narrow 
neck-like pedicel, and still enveloped by the thin membrane or follicle 
derived from the basal membrane. 
While still forming a part of the actual ovarian epithelium, the 
developing ova begin to show aggregations of yolk granules in their 
cytoplasm and it is to this accumulation of yolk granules, rather than 
the enlargement of the nucleus, that the rapid increase in size is due. 
The nuclei of such young ova are more or less spherical in form and rich 
in chromatin and surrounded by a pale and finely granular cytoplasm. 
(See Plate XXVI, fig. 4.) The extra-ovarian ova often assume a more 
or less pear-shaped form, the smaller end being the point of attach¬ 
ment to the pedicel. The whole of the cytoplasm is densely crowded 
with yolk granules, and the nucleus, w T hich attains a considerable size, 
generally presents an oval or elliptical form, is relatively poor in 
chromatin, and contains a large deeply-staining nucleolus (see Plate 
XXVI, figs. 2, 3). 
Up to this stage, the ovum is devoid of a shell and the idea that the 
shell is the product of the thin membranous ovarian follicle is quite 
untenable 1 . A portion of the wall of the ovarian tube is formed of cells 
which present a gland-like appearance (Plate XXVI, fig. 2, gl. ov.) 
and it is possible that this ovarian gland secretes the shell of the 
ovum. On the other hand, we have never seen spermatozoa at a 
higher point in the female genital tract than the upper part of the 
oviduct, and even then they have been isolated examples, the great 
majority of the spermatozoa remaining in the uterus and the lower 
part of the oviduct. Owing to the apparent impossibility of a sperma- 
tozoan penetrating the stout chitinous shell of the mature ovum, 
and, so far as we know there is no micropyle, fertilisation of the ova 
must be accomplished prior to the formation of a shell, in which case 
1 See Heller, C. (1858), p. 315. 
