THE GROWTH OF THE OVUM 



39 



110- 



100 



90- 



60- 



50 



The data of Aral (1920a) give a slight indication of the rate 

 of growth of ova since his tables show no ova above 20 /z 

 in diameter in 1 day rats, the first appearance of 20 to 40 ijl 

 in ova in 3 day rats, the first appearance of 40 to 60 /jl ova 

 in 10 day rats, and ova over 60 ijl in rats over 15 days of age. 

 Thus it may be inferred that growth to full size is attained 

 in a little over two weeks. Whether this time is taken also 

 in adult animals is not 

 known exactly, but 

 the minimum period is 

 at least ten days since 

 irradiated mice pro- 

 duce fertile eggs up to 

 ten days after irradia- 

 tion (Parkes, 1926- 

 27). This unplies that 

 there is a sensitive 

 period to x-irradiation 

 in young ova. Mar- 

 shak (1935) has shown 

 that the pachytene 

 stage of meiosis is es- 

 pecially sensitive to 

 x-rays, and young ova 

 enter into a modified 

 pachytene shortly 

 after leaving the ger- gj^^h^of 

 minal epithelium. mammals. 



Not all ova grow to '^^^"^^^•^ 

 mature size. This is evident at once from Aral's data which 

 show that an average of 0.8 per cent of the ova under 20 /x 

 in diameter attain a diameter greater than 60 fi, and only 

 2.7 per cent reach 20 to 40 jj. in diameter. The factors con- 

 cerned in the atresia of young ova are as unknown as those 

 determining their growth. 



The absolute size attained by mature ova varies from 

 species to species, but the limits are rather narrow (espe- 



o 

 o 



w 



H 

 W 



O 40- 



30 



20 



1 1 1 



100 200 300 



DIAMETER OF FOLLICLE, n- 



Same as Fig. 7, showing comparative 



the ovum in the various species of 



(From the Proceedings of the Royal 



