DEVELOPMENT OF THE OPOSSUM 89 
SUMMARY 
1. Several thousand eggs were removed from several hundred 
pregnant and pseudopregnant animals during the collecting 
seasons 1914 to 1917, an average of 11.5 per litter, or 23 from 
each animal. 
2. At least one-third of the average litter of eggs are unfer- 
tilized or abnormal (table 1). 
3. Six hundred and forty-one normal eggs, from tubal ova 
to the bilaminar blastocyst, form the basis of the present study. 
4. Collection of embryological material from the opossum has 
become greatly facilitated because of the discovery that the 
mammary glands of this animal hypertrophy at the approach 
of ovulation, so that the sexual condition of the female may 
be predicted with a high degree of certainty, without sacrificing 
the animal or without loss of time and effort, by simple though 
trained and practiced palpation of the glands. But the behavior 
of the mammary glands as well as the other reproductive organs 
is the same in the early stages of pseudopregnancy and in 
- pregnancy. Ovulation is always spontaneous. 
5. A series of photomicrographs of eggs in the living state is 
presented in the plates 1 to 11. 
The development of ten litters for given periods of time is 
shown in plates 1 and'2 and in figures 1 and 4, plate 9. 
Plates 12 and 13 are intended to serve as a résumé of the 
stages covered by the present study. 
The development of the opossum egg is illustrated in one 
series by the photographic plates 3 to 10 and in another series 
by the drawings plates 14 to 22. 
Six hundred different opossum eggs are shown, including 240 
illustrations of some 180 different preparations. 
6. The rate of development was determined in a number of 
cases in which the eggs of the right uterus were allowed to 
develop a given period of time after the removal of the left 
uterus and its contents. This method contributed in no small 
part to the success in securing an unbroken series of stages. 
7. The stages covered in this paper comprise about the first 
