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



and at each visit selected, with the aid of Mr. Cooper and his assistants, 3 sows 

 which were evidently in active heat. These were marked by ear-tags and isolated 

 from the herds in a special pen. The date of cessation of cestrus was noted by 

 report of the piggery laborers and by personal observation on the days of visit. 



At the end of 3 weeks 22 animals had been thus isolated, forming a series of all 

 stages of the cycle. These were then purchased by the University and resold to a 

 packer near the laboratory, by whom all of them were killed on the same day, 

 and the internal genitalia were thus recovered for study. The sows were all in 

 good condition, readily passing government inspection. They were of various 

 breeds and crosses, but were nearly uniform in age (about 10 to 12 months) averag- 

 ing over 180 pounds in weight, indicative of fairly mature state. 



These 22 animals have served as a basis for all statements in the following pages 

 as to the time relations of the reproductive cycle, but, owing to the variability of 

 the intercestral period, it chanced that none of them was killed within the first 

 3 or 4 days following ovulation. This gap in the series of uteri might have been 

 filled by specimens from the first collection of animals mentioned above, but un- 

 fortunately all the uterine preparations of those sows had been put permanently 

 beyond the reach of study by the departure for the war of a pupil to whom they had 

 been intrusted. For this reason a further series of 30 specimens was collected from 

 the slaughter-house. These were not obtained from animals that had been ob- 

 served during life, but were at least approximately dated by the fact that early 

 corpora lutea were present in all and that the ova or early embryos were recovered 

 from each specimen. By the microscopic structure of the corpora, as worked out 

 from the previous specimens, and by the condition of the ova, it was possible to 

 rank them in a series and to discover from them the earlier uterine changes fol- 

 lowing ovulation. 



It is obvious that there is a limit to the chronological accuracy of data obtained 

 in the various ways described above. Students of the human ovary and uterus 

 may perhaps think enviously of the opportunity afforded in the packing-house 

 and stockyard to collect fresh organs, datable more or less closely, from young, 

 healthy, mature animals to the number of twelve score and to see similar but 

 undated specimens totaling almost 12,000; on the other hand, no such exactness is 

 possible with the large animals of commerce as with animals like the rat, where the 

 method of Stockard, as applied by Long and Evans, permits prediction of the time 

 of ovulation within one hour. In this series of swine it has not been possible, in 

 most cases, to observe cestrus from start to finish, nor do we yet know more than 

 approximately the relation of the moment of ovulation to the usual 3 days of cestrus. 

 Moreover, we are dealing with variable quantities in the duration of the inter- 

 cestral period and the rate of development of the fertilized ova, both of which fac- 

 tors have been used in establishing correlations. However, the reader will prob- 

 ably not be led astray if we estimate that all references to a given day, assuming 

 a 21-day cycle as a working basis, imply a possible error of == 1 day, perhaps even 

 of 2 days as we approach the latter days of the intercestral period. 



