CYCLIC CHANGES IN THE OVARIES AND UTERUS OF THE SOW, ETC. 131 



scantiness of fibroblasts of the usual type, the presence of so complete a reticulum 

 gave cause for surprise until it was found (Corner, 1920) that here, as in a number 

 of other organs, the capillary endothelial lining is the source of the reticulum. As 

 the capillary bed is so complex that every cell in the gland is in contact with a 

 blood-vessel, it is not difficult to understand the great density of the reticulum. 



RETROGRESSION OF THE CORPUS LUTEUM. 



It is a fact of the greatest importance that the changes, just summarized, 

 which lead up to the formation of the corpus luteum, take place with equal complete- 

 ness, whether or not the sow has been impregnated. It is not possible to distinguish 

 the corpus luteum of pregnancy from that of unfertilized ovulation during the first 2 

 weeks after discharge of the ova, nor can the observer determine, from the appear- 

 ance of two specimens of equal age, which of them was destined to further growth 

 in size and a 4 months' span of activity, and which to immediate retrogression. 



The time of degeneration of the corpus luteum of unfertilized ovulation can be 

 determined with fair accuracy, for the data, though few, are in complete accord 

 with each other. Seven animals of our series were taken 15 or more days after an 

 observed oestrus but before another ovulation had occurred, and all of these con- 

 tained retrogressing corpora lutea, while all those killed earlier showed no sign of 

 degeneration; 2 of the 7 had begun a second cestral period and thus gave a further 

 check upon the time relations. It is upon the fourteenth or fifteenth day, then, 

 that a sudden change overtakes the structure which has been so elaborately erected. 

 Within 2 or 3 days the diameter of the corpus has decreased from 8.5 or 10 mm. 

 to 6 mm.; its color has changed from the pink of an active capillary circulation to 

 the whitish tone of scar-tissue, and its texture has much increased in firmness and 

 toughness. Microscopic examination (fig. 6, pi. 1; fig. 14, pi. 2) shows the change 

 to have been so rapid as almost to defy comparison with the previously existent 

 conditions, and exact analysis of what has happened is thus rendered difficult. An 

 example of the suddenness of the break-down is given by one of the animals, killed 

 on the fifteenth day, in which some of the corpora were degenerated while others 

 were intact, but in one corpus there was a patch of advanced degeneration sur- 

 rounded by unchanged tissue. 



The most striking feature of the change is the disappearance of the granulosa 

 lutein cells. In the sows whose ovaries show the least degeneration many of these 

 cells can still be distinguished in various stages, from a fair state of preservation 

 to almost complete degeneration (fig. 14, pi. 2), but in other animals of this 

 period nothing remains but vague vacuolar spaces containing many pycnotic 

 nuclei and nuclear fragments, where but a few days before were closely packed 

 masses of cells, among the most imposing of the animal body. Since these cells 

 in number and bulk formed by far the greater part of the corpus luteum, it is obvious 

 that their breakdown alone will account for the decrease in size and for the relatively 

 greater density of the retrograding corpus. Another result of the degeneration is 

 seen at the periphery of the corpus luteum, where the former sharp demarcation 



