DEVELOPMENT OF THE OPOSSUM ct 
pl. 12, figs. 13, 14, 15, pl. 17 and accompanying text fig. 2). Measure- 
ments in salt solution and in fixing fluid are as follows: 
In Ringer’s solution (average 0.66 mm. and 0.34 mm.). 
‘Rhroueheshelle seen On7s) (0268 0265, 0065) 0.65 0.60 0.65 0.65 
Through blastocyst........ 0.35 0:35 0:40 0.37 0.35 0.30) 0.32 0.30 
In fixing fluid (average 0.57 mm. and 0.33 mm.). 
Hill’s fluid Aceto-osm.—biochr 
ithroueheshelle ener On COR SonOsoon Room: OFG0R OL50ORGOM O55 0s54 
Through blastocyst...... O34 10:32) 0533 0.3) 0733 10.34 10.32 0.35 0.34 
No. 203. Received Jan. 26. Removed only left uterus, leaving 
ovary, Jan. 28, 8:40 a.m.; about 12 eggs: one with pronuclei (fig. 20, 
pl. 14; some 2-celled (text Wed hy tOrd wud. spl. bss hou plik) s 
one 3- celled (text fig. 4, 7); others 4-celled (fig. 8, pl. 13: ; fig. 7, ple 15); 
one recorded measurement of whole egg is 0.44 mm. ‘through shell 
membrane, 0.15 through ovum. 
OGOO oO © 
Fig. 2. Five blastocysts with embryonic areas and one unfertilized egg of 
litter No. 194’, sketched alive in Ringer’s solution with the aid of the camera 
lucida. X 8. 
No. 203’. Second operation at 11:45; date not recorded in protocol, 
but cage record indicates that the time was 11:45 p.m., Jan. 29; hence 
the interval was probably 39 hours; removed only right uterus, leaving 
both ovaries. Several young vesicles of about 50 cells; one measure- 
ment in salt solution is 0.5 mm. through shell membrane, 0.16 mm. 
through ovum. Killed Feb. 16; corpora lutea had almost entirely 
disappeared, follicles still small, but mammae very thick as in 
pregnancy. 
No. 205. Captured by dogs Jan. 28, the skin being ripped at 
shoulder; operated Jan. 29, 9:15 p..; 10 eggs: 9 young bilaminar 
blastocysts with much albumen at one ‘pole: entoderm quite or nearly 
reaching non-formative pole; three measured in salt solution 1.05 mm. 
three 1 mm., two 0.90 mm., and one 0.75 mm.; one unfertilized egg 
measured 0.72 mm.; in alcohol after two years the eggs measured about 
O70rmmra hes, 29° Ti and-12,-pl. 19): 
No. 205’. Killed Jan. 30, 10:10 a.m. (interval 13 hours); 13 eggs: 
3 unfertilized, the remainder vesicles with faint primitive streak in 
rounded areas or with more advanced primitive streaks in pear-shaped 
areas. Two of the former measured in alcohol 1.45 and 1.83 mm. 
with areas 1.1 and 1.2, Tespectiv ely; one of the latter 2 mm. with area 
1.32 x 1 mm. 
