DEVELOPMENT OF THE OPOSSUM 9 
No. 82. Bilaminar blastocysts like those of No. 50 (figs. 1 and 1A, 
pl. 20). 
No. 88. Four 4-celled eggs (figs. 8, 11, and 12, pl. 15) and three 
young blastocysts! (figs 12 and 19, pl. 16). 
No. 85. Cleavage stages; one each of 6, 7, 9, 10, 12, 14, 15, 17, 
and 18 cells; three of 8 cells; five of 16 cells (figs. 15 and 17, pl. 15). 
No. 88. Of these eggs the collection contains twenty-seven excellent 
preparations consisting of 50 to 70 cells and ranging up to 103 cells 
each. Most of the eggs have from one to several entoderm mother 
cells in their earliest proliferation (figs. 2, 4, and 6, pl. 7; figs. 3, 7 
to 11, 18, 21, and 22, pl. 16; fig. 13, pl. 22; compare also page 36, 
Hartman, 716). 
No. 94. <A single bilaminar blastocyst about 1 mm. in diameter. 
No. 112. Degenerating ova from known second oestrus period. 
No. 117’. Forty-three eggs, mostly in cleavage, 2- to 16-celled, from 
a single uterus, the organs on the opposite having been removed 33 
days before; ovary hypertrophied; eggs subnormal in size (figs. 9 
and 10, pl. 15). 
No. 144. Blastocysts more advanced than those of No. 88; at- 
tenuation of non-formative area well under way (figs. 1 to 3, pl. 17). 
QOOOo®O 
Fig.1. Three blastocysts and one unfertilized egg of litter No. 175’, sketched 
with the aid of the camera lucida immediately upon immersion in the fixing 
fluid (aceto-osmic-bichromate). X 8. 
No. 173. Received Jan. 17. Left uterus and ovary removed 8 
p.m., Jan. 18; about 12 eggs, of which 8 were sectioned: 7 are 4-celled 
(fig. 5, pl. 3) and one is 3-celled (text fig. 4, L; fig. 8, pl. 15). 
No. 173’. At 8 p.m., Jan. 19 (interval 24 hours) about 12 just com- 
pleted blastocysts were secured from right uterus; no entodermal 
mother cells present (fig. 5, pl. 7). Killed Feb. 9, when the completely 
hysterectomized, semi-spayed animal was again coming into heat. 
No. 175. Received Jan. 17; removed left ovary and uterus; pseudo- 
pregnant; the degenerating eggs were not counted or preserved. 
No. 175’. Removed male Feb. 9; killed Feb. 14 (interval 28 days 
after operation); 14 eggs: 6 unfertilized, 8 very attenuated vesicles, 
entoderm reaching almost to equator (fig. 8, pl. 18; figs. 7 and 7A, 
pl. 19 and accompanying text fig. 1). The measurements of the eggs 
of litter 175’ are here given as made in salt solution: 
1 This is the only instance in all of my records in which the eggs, all removed 
at the same time from the animal, consisted of two distinct groups or stages, 
separated by a considerable period of development, in this case about twenty- 
four hours. There is, of course, a possibility of error on my part due to mixing 
of labels in this case. 
