148 



Richard M. Fraps 



In view of the consistent interruption of ovulation following placement of 

 lesions in the ventral preoptic region, experiments were undertaken toward 

 further delimitation of the effective locus. This, as described by Ralph, "is 

 just dorsal to the optic chiasma, at the extreme rostral end of the hypothala- 

 mus, lies laterally less than 2 mm from the midline, and within a ventral por- 

 tion of the nucleus praeopticus paraventricularis". Lesions of 1 to 2 mm 

 diameter, placed within this region 0.5 to 1 mm on each side of the midline, 

 regularly prevented ovulation, while the response to smaller lesions so placed 

 was irregular. 



The effects of hypothalamic lesions on progesterone-induced ovulation of 

 the Ci follicle were also investigated (51, 52). Progesterone was administered 



m nn 



Fig. 3. Mid-sagittal plane of chicken diencephalon on which certain hypothalamic nuclei 

 and the sites of all lesions made within 2 hr following administration of progesterone are 

 projected. Symbols indicate the approximate center of each lesion: O — ovulation; 

 A — failure of ovulation. 1 and 2 — principal and accessory parts of preoptic paraventricular 

 nucleus, 3 — lateral hypothalamic nucleus, 4 — mammillary nucleus, 5 — tuberal nucleus, 

 6 — median eminence, 7— optic chiasma, 8 — anterior commissure, 9 — posterior commissure, 

 10 — cerebellum. Fiber tracts not shown. Limits of nuclei are somewhat arbitrary. 

 From Ralph and Fraps (52). 



