638 



Fishery Bulletin 103(4) 





SY 



LP 





Figure 3 



Histology of the ovarian maturity and oocyte developmental stages of Rikuzen sole tDexistes rikuzenius). 

 (A) Spent phase. Bar: 200 ^im. (B) Middle vitellogenic phase. Bar: 200 jim. (C) Late vitellogenic phase. Bar: 200 um. 

 (D) Maturity phase. Bar: 200 urn. (E) Oocyte at the cortical alveolus stage. Bar: 50 um. (F) Oocyte at the premature 

 stage. Bar: 50 urn. EP=early perinucleolus stage, LP=late perinucleolus stage, CA=cortical alveolus, PY=primary 

 yolk stage, SY=secondary yolk stage. TY=tertiary yolk stage, MN = migratory nucleus stage, AT=atretic oocyte, 

 POF=postovulatory follicle. 



relatively low, less than 2.0, from January to March, 

 increased steeply from April to August, and progressed 

 to more than 15.0 during September to October. Values 

 then rapidly decreased from October to November. The 

 BC was low from January to April, increased to a maxi- 

 mum value of 18.3 in August, and then decreased to 

 13.7 in November. 



Histological observations of oocyte development 



Although oogenesis is continuous, in order to explain 

 the developmental process, oocyte development was 

 divided into eight stages, basically according to Yama- 

 moto (1956) (Fig. 3). The characteristics of oocytes, cell 

 and nuclear diameters, and time of occurrence of each 

 stage of oocyte and POF are shown in Table 1. 



Maturity 



Ovary maturity did not differ among positions in the 

 ovarian lobe or between eyed-side and blind-side lobes. 

 In addition, the diameters of the largest oocytes did 

 not vary significantly among positions (ANOVA, F 2 42 = 

 0.354, P=0.704) or between lobes (paired t-test, /=14, 

 r=0.058, P=0.955). Therefore, maturity was determined 

 from observations of the middle portions of eyed-side 

 ovaries. 



Maturity was classified into eight phases, the charac- 

 teristics of which are shown in Table 2. Because oocytes 

 younger than the late perinucleolus stage occurred 

 throughout the year, maturity was determined as oc- 

 curring from this phase onwards. GSI values signifi- 

 cantly varied among maturity phases (ANOVA, F 4205 , 



