420 



IMPLANTATION OF THE OVUM IN 



females are at about the same stage of gestation. While the later stages of preg- 

 nancy are quite easily obtained, it has taken much labor in the field at the beginning 

 of the breeding season for a number of years to secure a complete series of stages. 

 The preplacental stages have been by far the most difficult to secure, as there are 

 only a few days in each season in which they can be obtained. During this period 

 the majority of the animals killed are males, the females for the most part remain- 

 ing under ground. Again a day or so of cloudy or rainy weather at this particular 

 time may almost ruin the chances of securing early stages for that year. However, 

 the fact that they carry a large number of young is a compensating feature. 



The following tables may prove of interest in showing the number and distribu- 

 tion of embryos in the uterus. They are compiled from notes based on collections 

 made at the height of the season of 1900, an unusually favorable year for this purpose. 



TABLE I. 



NUMBER OF EMBRYOS PER ANIMAL. 



TABLE II. 



DISTRIBUTION OF EMBRYOS IN RIGHT AND LEFT 

 HORNS OF UTERUS. 



It will be seen from these tables that the 129 pregnant females averaged nearly 

 8.5 embryos with a total variability of from 5 to 13. As regards the distribution in 

 the two horns of the uterus, the right horns had 537 embryos and the left 557, averag- 

 ing over 4 for each horn, the range of variability being the same, from to 9 for each 

 side. 



This record is based on one week's collecting during the middle of May, all the 

 embryos being fairly well advanced. Only 4 non-pregnant females were taken during 

 this week, showing that most of the females are pregnant at this time. It is also 

 interesting to note that in 14 cases some one embryo was abortive, and, acting as 

 a plug, effectually prevented the delivery of the remaining embryos in the ovarial 



