ORIGIN OF THE CORPUS LUTEUM 13] 



elude previous experience of the author with the ovaries of swine; 

 certain presumed histological advantages of the species, to be 

 explained later, and, above all, the fact that the previous work 

 on swine has been much quoted by writers in confirmation of the 

 thecal-origin theory. Here, if anywhere, the application of 

 modern methods of research should settle the old difference once 

 for all. 



MATERIAL AND METHODS 



As it has been pointed out that the only hope of trustworthy 

 results in this problem depends upon the possession of an un- 

 broken series of normal specimens of known ages, a description 

 of the material in the author's hands and the methods of obtain- 

 ing it will be given in detail. The first step was a preliminary 

 investigation to obtain exact knowledge of the period in the 

 reproductive cycle at which the ova are shed from the ovary in 

 order that mature follicles and very early corpora lutea might 

 be obtained. The results of this study have already been pub- 

 lished (Corner and Amsbaugh, '17) and will not be repeated in 

 detail here. We were able to confirm the current supposition 

 that in swine ovulation is coincident with the oestral period, 

 and by this fact we are at once provided with the means of 

 obtaining the desired stages of corpus luteum formation. 



The females of the wild swine of Europe are monoestrous, 

 according to Kaeppeli ('08), having but one period of heat in the 

 year; but under domestication the sow becomes polyoestrous, 

 coming in heat at intervals of two to four weeks, usually about 

 every twenty-one days, as all breeders agree. The period of 

 heat commonly lasts three days and is characterized by sexual 

 excitement and in some individuals by swelling, reddening, and 

 slight eversion of the vulva, or even at times by a serous, mucous, 

 or partially sanguineous discharge from the genital orifice. If a 

 boar be present, the sexual excitement is made apparent by 

 ready acceptance of coitus (commonly on the second or third 

 day of oestrus) ; if none but females are in the pen, the sow in heat 

 will be seen to sniff at the genitals of her neighbors and 'ride' 

 them in imitation of coitus. Frequently the sow is the recipient 



