50 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



the ovum. The zona radiata is transparent and 

 sculptured with very fine striations on its outer 

 surface. This membrane is clearly evident at all 

 stages of maturation. Formalin-preserved ova in 

 this group are semiopaque and are barely distin- 

 guishable without magnification. Fresh ovaries 



made of all ova above 0.18 mm. in diameter in the 

 sample. Their diameter frequency distribution, 

 shown in figure 1, clearly indicates the presence of 

 several groups which simultaneously are develop- 

 ing to maturity. Two chief groups are distin- 

 guishable: Intermediate, from 0.18 to about 0.40 



IMMATURE 



INTERMEDIATE 



MATURING 



i 



o 

 85 



ID 



2 



1200 



I0O0- 



800- 



400 n 



200r 



OVA DIAMETER IN MILLIMETERS 



Figure 1. — Frequency polygon of 12,041 ova diameters measured from a maturing ovary of a yellowfin tuna taken in 

 Hawaiian waters, May 5, 1950. Only ova measuring more than 0.18 mm. in diameter were included. 



containing ova which have developed to the inter- 

 mediate size are indistinguishable in appearance 

 from those containing only immature ova. 



Maturing: As the ova advance toward maturity, 

 the group, beyond about 0.40 mm., becomes opaque 

 and exhibits a characteristic yellowish color in 

 formalin. The yolk granules appear as highly 

 refractive, spherical bodies in the cytoplasm. At 

 this stage of development, the fresh ovaries are 

 conspicuously turgid, yellowish in color, and the 

 ova are firmly embedded in the follicles. 



To demonstrate the size distribution of the in- 

 termediate and maturing ova groups, a sample of 

 tissue (0.5 gm.) was taken from a maturing ovary 

 of a fish landed in early May. After separation 

 of the ova from the follicles, measurements were 



mm. ; and maturing, beyond 0.40 mm., which itself 

 appears to consist of more than one group. 



Ripe: On reaching the stage at which the ova 

 are about to be spawned, distinct morphological 

 changes occur in them, and they lose their opacity 

 and assume a translucent, grayish color in forma- 

 lin. Ripe ova measure about 0.76 to 1.23 mm. in 

 diameter. Embedded in the yolk is a single, con- 

 spicuous, golden-yellow oil globule, which averages 

 about 0.26 mm. in diameter. Ripe ova readily 

 break from the follicles in which they develop, 

 collect in the lumina of the ovaries, and flow freely 

 from the oviduct when the sides of the fish are 

 pressed. 



To show further the size distribution of these 

 developmental groups, a frequency distribution 



