ORIGIN OF THE CORPUS LUTEUM 119 



pointed out that the arguments quoted above are based merely 

 upon analogies between the layers of the follicle and the fully 

 formed corpus luteum, and that from the writings of his prede- 

 cessors it is apparent that few had actually seen corpora lutea in 

 process of formation. Even when descriptions are given of 

 mature follicles or supposed early corpora lutea, there is usually 

 no proof that the structures in question actually represent the 

 results of normal follicular development or recent ovulation. The 

 problem should be worked out from a series of specimens gathered 

 at known periods after rupture of the follicle; and in order to 

 avoid confusion with atresia or other irrelevant processes, each 

 follicle or corpus luteum studied should be certified as to its 

 normal condition and stage of development by comparison with 

 the fertilized ovum or embryos proceeding therefrom. To fulfill 

 these high requirements calls for long and tedious labors — the 

 investigator must spend hours and days in observation of his 

 animals ; the reproductive cycle of the species used must be known 

 well enough to acquaint him with the time of ovulation, the ani- 

 mals must be killed at definite times thereafter, and the ova must 

 then be sought in the ovary, the oviducts, or the uterus. Sobotta 

 himself chose the mouse, in which he had found that an ovulation 

 takes place about twenty-one days after the birth of a litter, and 

 in which the small size of the animal permits serial sectioning 

 of the entire ovaries and Fallopian tubes. It must be admitted 

 that his own postulates could not be followed to the full; the in- 

 dividual corpus luteum corresponding to a given ovum cannot be 

 identified, because many ova are extruded at one ovulation in 

 this species; the exact time of ovulation may vary by hours, and 

 again there is so much variation of the interval between ovula- 

 tion and the entrance of the spermatozoon into the egg that the 

 condition of the ovum cannot be used as an exact measure of the 

 age of the corpus luteum. Study of the ova merely provides 

 assurance that the corpora lutea are normal and gives a rough 

 means of determining their ages. The judgment of the investi- 

 gator must finally be used to rank the corpora lutea in an orderly 

 series. It is beyond denial, however, that Sobotta possessed 

 such a series, collected from about 200 mice and based upon the 



