CYCLIC CHANGES IN THE OVARIES AND UTERUS OF THE SOW, ETC. 129 



II. OVARY. 

 ORIGIN AND COMPLETED STRUCTURE OF THE CORPUS LUTEUM. 



The author's views as to the origin of the corpus luteum have been fully con- 

 firmed by the examination of new material obtained in connection with the study 

 of the uterine cycle, amounting to about 35 animals taken in the first 8 clays after 

 ovulation. For a detailed description of the changes undergone by the discharged 

 follicle during its conversion into the corpus luteum, the reader is referred to the 

 author's article of 1919; at the present time a brief summary will suffice as intro- 

 duction to an account of the subsequent history and degeneration of the corpus 

 luteum. 



When the Graafian follicles collapse at the time of rupture, the extrusion of the 

 follicular fluid and the contraction of smooth-muscle cells in their walls reduce their 

 diameter of 8.5 to 10 mm. to 4 to 6 mm. By the end of one week's growth, however, 

 the corpora lutea are again 8 to 9 mm. in diameter, and if pregnancy ensues there 

 is a further growth until, after 2 or 3 weeks, the maximum size of 10 to 11 mm. is 

 reached. The membrana granulosa is retained intact, except for loss of the cumulus 

 oophorus, after rupture of the follicle. Its cells increase in size without division, 

 their cytoplasm becomes laden with lipoid substances, and they become the larger 

 elements, commonly knowfi as "lutein cells," in the fully formed corpus luteum. 

 During this process the membrana granulosa is invaded by blood-capillaries from 

 the theca interna, which ramify to" form an extensive vascular plexus throughout the 

 new structures. The large lipoid-latlen cells of the theca interna of the Graafian 

 follicle are increased in number by mitotic divisions and pass into the corpus luteum 

 to become lodged between the granulosa cells throughout the whole structure. 

 There is no evidence that the cells of the theca interna are ever converted into 

 fibroblasts of the usual spindle-cell type or that they lay down the fibrils of the 

 close-meshed reticulum which is present in the corpus luteum. 



The time relations of these changes may be given as/ollows': ovulation probably 

 occurs on the second day of oestrus ; invasion of the granulosa by blood-vessels and 

 theca interna cells begins on the third or fourth day after on^et of osstrus and is 

 completed about the sixth or seventh day; by the seventh day the corpus luteum is 

 usually solid and its cells have become fully differentiated. 



Kiipfer (1920) has given a valuable series of colored plates representing the 

 gross appearance of the follicles and of the corpora lutea at various stages of develop- 

 ment and retrogression. Unfortunately, the figures were not made from specimens 

 of known relation to the cestrous cycle, and for this reason we have provided (plate 2) 

 drawings of ovaries of 3 sows killed at known periods : during astrus (fig. 9) ; at 

 8 days after ovulation, with completely solidified corpora lutea (fig. 10); about 17 

 days after ovulation, with degenerating corpora lutea and a new group of follicles 

 beginning their precestrous enlargement (fig. 11). 



The corpora lutea of a sow at the tenth day after ovulation are very conspic- 

 uous objects by reason of their size, reaching 8 to 9 mm. in diameter and pro- 

 jecting nearly all their bulk from the ovary, so that when numerous corpora are 



