BREEDING HABITS OF THE OPOSSUM — HARTMAN. 355 



the former almost double the latter. To make another comparison, 

 our No. 114 yielded embryos of 8-10 somites eight days after copu- 

 lation while No. 343 furnished only 1 mm. vesicles (fig. 36, pi. 8) in 

 about the same time, a calculated discrepancy of nearly three days. 

 Figures on the total period between copulation and birth show 

 the same discrepancies. Thus Hill gives one case of parturition 8 

 days, one of 16 days and a third of 20 days after the last copulation. 

 Selenka's opossum gave birth 12 davs, 20 hours after copulation. 

 With this the single specimen which the writer allowed to come to 

 term closely agrees. 



RATE OF DEVELOPMENT. 



The writer has, however, considerable data on the rate of develop- 

 ment, inasmuch as two stages were secured from each of a large 

 number of females. Thus, from No. 306 2-celled eggs were taken at 

 a given time from the left uterus and 116£ hours later 1 mm. vesicles 

 were taken from the remaining uterus. No. 360 yielded 1 mm. 

 vesicles from the left uterus and 70 hours later embryos with small 

 allantois, about three days of term, were removed from the surviving 

 organ. Adding these periods together we have a total calculated 

 period of gestation of about 11 days. 



Similarly the eggs of our No. 293 developed in 3^ days from the 

 4-celled stage (fig. 14, pi. 4) to vesicles (fig. 15), about a half day 

 more advanced than the first eggs taken from No. 585 (fig. 16, pi. 5) ; 

 and these in turn developed h% days longer, arriving at the stage of 

 embryos about 2£ days of term (fig. 17, pi. 5). These periods like- 

 wise total between 11 and 12 days. Longer intervals are represented 

 by Nos. 342 and 561. In the former, 30- to 50-celled eggs became em- 

 bryos about 3 days of term in 7 days ; this record indicates 11 days 

 as the approximate period of gestation. In the case of No. 561, 

 Figures 18 and 19 (pi. 5) represent 7£ days of development. This 

 points to about 10 days as the period of gestation. 



From the data thus far in hand it appears that 11 days is very 

 near the average period of gestation. The possibility is not excluded 

 that this period varies somewhat and that the variability in the time 

 between oestrus and birth is to be explained in the varying length of 

 both the postcestrous period and the period of gestation. 



PASSAGE THROUGH THE FALLOPIAN TUBE. 



Twenty-four hours or a little less are required for the eggs to 

 pass through the Fallopian tube. This estimate was first made on 

 the basis of the rate at which the albumen is deposited ; later this was 

 found to be substantially correct through a successful attempt to 

 "mark" the eggs. Into the body cavity of female No. 566, which 

 had just ovulated, were placed several score of Ascaris lumbricoides 



