104 John W. Everett 



with a precision microammetcr and continuous d.c. current. Unless otherwise 

 stated it will be understood that the stimulus consisted of monophasic, 

 1-nisec pulses, at a frequency of 100/sec, in 30-sec trains at 30-sec intervals. 



On the morning after stimulation the reproductive tracts were removed 

 under ether anesthesia. The ampullas and adjacent turns of the oviducts 

 were microscopically explored for ova (9). The ovaries were examined in 

 physiological saline under the dissecting microscope. When fewer than 7 or 8 

 ova appeared in the ampullas, special attention was paid to the ovaries for 

 signs of partial activation. No rigorous histological search was made, however. 

 The brain was fixed by perfusion of the head with 10"o formalin in saline. 

 Paraffin sections were cut at 25 micra and these were subsequently stained 

 by the Luxol fast blue-cresyl fast violet technique. 



EXPERIMENTS AND OBSERVATIONS 



Exploratory Studies 



The early phases of the investigation were reported briefly to the Endocrine 

 Society (11). These experiments are of interest at the present time largely 

 because they eventually led us to explore the region between the rostral 

 margin of the optic chiasma and the anterior commissure. The technique 

 was at first relatively crude: the electrodes were comparatively coarse and 

 stimulation parameters were quite different from those described above. 

 The pulse duration was unnecessarily long (15 msec at 30/sec), as was the 

 over-all period of stimulation, alternate 30 sec for 30 min. Voltages were 

 not excessive, ranging from 3 to 5. 



There were 29 proestrous rats in which electrode tips rested within 2 mm 

 above the floor of the diencephalon or preoptic region (Series I). Sixteen 

 rats ovulated during the following night. In 6 cases the electrodes lay in the 

 preoptic region and 5 of the 6 ovulated, a fact that was especially interesting 

 because of the implication from Critchlow's study that the electrodes must 

 be close to the median eminence to be effective under pentobarbital anesthesia. 

 Figure 1 shows the preoptic nucleus projected on the mid-sagittal plane of 

 the rat brain. 



Advanced Ovulation in Diestrous Rats 



In rats that experience regular 5-day cycles the time of ovulation can be 

 advanced 24 hr by injection of progesterone on the third day of diestrus (9). 

 This effect, like spontaneous ovulation, is subject to blockade by atropine and 

 is presumably mediated through the central nervous system (12). The latter 

 study demonstrated that during the afternoon of diestrus day 3 there is a 

 limited period of sensitivity to progesterone approximately 24 hr in advance 

 of the "critical period" on the day of proestrus. There was thus excellent 

 reason to expect that electrical stimulation of the hypothalamus in late 



