160 THE PHYSIOLOGY OF REPRODUCTION 



the other kitten was very small, and apparently about three 

 weeks developed. 



FORMATION OF OVA 



It is usually stated that all the ova which are to be de- 

 veloped in the ovary exist in it at the time of birth, and that a 

 considerable proportion of these undergo atrophy before puberty. 

 Thus, the number of ova in the ovary at birth has been estimated 

 at 100,000, of which it is supposed that not more than 30,000 

 remain at puberty. 1 



Miss Lane-Clay pon, 2 however, has described the formation of 

 ova, resembling primordial ova, from interstitial cells during 

 adult life. These cells are shown to increase markedly in size, 

 their length being often almost doubled. In addition to their 

 becoming enlarged, certain of the interstitial cells near the 

 periphery undergo further changes during the later stages of 

 pregnancy. The cells appear to pass outwards and become cut 

 off by connective tissue, and in many cases almost reach the 

 surface of the ovary. This process begins in the rabbit at 

 about the twentieth day of pregnancy. A little later some of 

 the cells appear to be multi-nucleated, and it is suggested that 

 these are formed by the fusion of the same number of inter- 

 stitial cells as there are nuclei. The nuclei then degenerate 

 with the exception of one, and the inference is drawn that the 

 latter lives and grows at the expense of the others in just the 

 same way as Balfour concluded that one developing ovum in 

 the immature ovary might be nourished by the surrounding 

 ova which were undergoing degeneration. 



In the ovary of a rabbit whose time of parturition had nearly 

 arrived, the interstitial cells were observed to have undergone 

 further changes identical with those taking place in the deuto- 

 broque cells of a young ovary during the period of oogenesis (see 

 above, p. 120). The leptotenic stage is rapidly passed through 

 and the nucleus enters upon the synaptenic condition, which 



1 Galabin, A Manual of Midwifery, Gth Edition, London, 1904. Accord- 

 ing to another calculation the human ovary at the age of seventeen contains 

 17,600 ova (Heyse, Arch. f. Oynak., vol. liii., 1893), of which only 400 

 become mature (Henle, Handbuch der Anatomic, 1873). 



2 Lane-Claypon, " On the Origin and Life-History of the Interstitial Cells 

 in the Ovary in the Rabbit," Proc. Roy. Soc., B., vol. Ix.xvii., 1905. 



