CYCLIC CHANGES IN THE OVARIES AND UTERUS OF THE SOW, ETC. 121 



The number of young in a litter varies from 1 to 23, the mean being from about 

 7 to 9 in different American breeds, according to Surface (1909), who has made a 

 biometric analysis of the data upon this subject. 



COLLECTION OF THE MATERIAL. 



The first step in correlating the organic changes with the outwardly visible 

 functional events which have been recounted is, necessarily, to determine at what 

 point of the cycle ovulation takes place. The results of this part of the investiga- 

 tion, and of a study of the origin of the corpus luteum and its alterations during 

 pregnancy, have already been described in previous publications by the writer 

 (1915, 1917a, 19176, 1919) and need only be summarized here. 



It was found, as had been expected by analogy with previously known species, 

 that the period of oestrus is the time of ovulation. In the stockyards the condition 

 of heat was observed in about 30 animals, which were suitably marked and then 

 were followed through the processes of the abattoir until the genitalia were ob- 

 tained from the butcher's hands. In such of these animals as were killed during 

 the three days following the onset of cestrus the ovaries contained either mature 

 or recently collapsed Graafian follicles. This much had in fact already been 

 proved by Lewis (1911) in a publication at the time unknown to the writer, but 

 we were able to strengthen the evidence by actual recovery of the ova from the 

 Fallopian tubes. By the use of simple technical methods, which will be described 

 later, a series of segmentation stages and also of unfertilized ova was obtained 

 for study and correlation with the corpora lutea. By good fortune the packing- 

 house where these first steps were taken was not at the time under great stress of 

 production; a few of the swine were retained in the corrals as long as 11 days after 

 the onset of cestrus, and it was therefore possible to follow the development of the 

 corpus luteum up to about the tenth day. It was not until the adoption of a rapid 

 method of locating ova and blastodermic vesicles in the large uterine chambers of 

 the sow that the fate of the ova after their exit from the Fallopian tubes could be 

 continuously followed; but the writer already had in hand the ovaries of sows in 

 all stages of pregnancy from the third week until parturition, to the number of 

 about 140. Upon the sum of this material two previous contributions were based 

 (1915, 1919), covering, therefore, the corpus luteum of pregnancy from the day of 

 ovulation until after parturition and the corpus luteum of unfertilized ovulation 

 until the tenth day of its development. 



It was, of course, impossible at the stockyards to follow animals into the latter 

 half of the cycle. Such material, however, was made available by the generous 

 interest of Mr. Walter N. Cooper, manager of the American Feeding Company, 

 of Baltimore, who extended every facility for the undertaking. At the establish- 

 ment of the company, about 20 miles from Baltimore, large numbers of pigs were 

 kept from an early age until they attained a profitable weight by the consumption 

 of table refuse and other edible garbage collected in the city. The writer made a 

 series of trips to the piggery farm on alternate days throughout a period of 3 weeks, 



