DEVELOPMENT OF THE OPOSSUM lex 
No. 339. Jan. 28, 3:30 p.m., 8 eggs (fig. 6*, pl. 9): 2 unfertilized; 
5 eggs with more or less abnormal, round, thick-walled blastocysts at 
one pole (fig. 2, pl. 6; fig. 15, pl. 18; figs. 5, 5A, and 6, pl. 19); one quite 
normal thin-walled blastocyst with entoderm spread to equator (fig. 2, 
pl. 6; figs. 6 and 6A, pl. 18). 
No. 339’. Killed 12:30 p.m., Jan. 29 (interval 21 hours); 9 eggs: 6 
bilaminar blastocysts measuring about 0.85 mm. in alcohol; one dead 
blastocyst 0.75 mm.; 2 unfertilized eggs (fig. 3, pl. 21). 
No. 342. Received Jan. 27; Jan. 28, 9:30 p.mM., 19 eggs; of the 4 
sectioned specimens two are defective and the other two are blastocysts 
of 26 and 28 cells, respectively (figs. 6* and 7*, pl. 5; fig. 20, pl. 15). 
No. 342’. Feb. 4, 8:15 p.m. (interval 7 days); 2 dead and 9 normal 
embryos, the latter about 7.5 mm., head-rump length. 
No. 343. Observed copulation, 4 a.M., Jan. 22; Jan. 29, 2:45 P.M., 
left uterus yielded 15 eggs: 4 unfertilized; one small defective blasto- 
cyst; one blastocyst with defective embryonic area; 9 normal bilaminar 
blastocysts about 1 mm. in diameter, embryonic areas 0.64 to 0.76 mm. 
Gi ple Zoaties o. pl. 21): 
No. 343’. Killed 7 hours, 20 minutes later (73? days after copula- 
tion); 8 eggs, of which 3 are unfertilized, 5 normal 1.8-mm.blastocysts 
‘just preceding proliferation of mesoderm; embryonic areas 1 to 1.1mm. 
in diameter (fig. 6, pl. 2*; fig. 2, pl. 22). 
No. 344. Received and operated Jan. 29, 4:45 p.m.; 16 eggs: 15 
sectioned; of these 6 are unfertilized and fragmenting; 2 are abnormal 
blastocysts (fig. 4, pl. 16); 7 are normal blastocysts showing early 
differentiation of embryonic and non-embryonic areas; the most ad- 
vanced contains 124 ‘ectodermal’ and 45 entodermal cells (figs. 5*, 6%, 
and27, pl. Si fies. 14 tor l7, pl. 16). 
No. 344’. Killed Feb. 1, 8:30 p.m. (interval 3 days, 32 hours); 7 
eggs, all normal vesicles 4 mm. or more in diameter, with short 
medullary groove. 
No. 346. Received and operated Jan. 29, 8:45 p.M.; 21 eggs: 8 
unfertilized; one dead blastocyst; 8 normal 1.5 mm. blastocysts, 
embryonic areas about 1 mm.; the other eggs retarded and defective 
(igs; ply 25): 
No. 346’. Killed next morning at 6:35 o’clock (interval 9$ hours); 
22 eggs: 11 unfertilized; 11 blastocysts ranging up to 2.2 mm. in 
te all in early primitive-streak stages (fig. 4, pl. 2*; fig. 22, 
pl. 13). 
No. 347. Jan. 29, 9:45 p.m.; 15 eggs: 4 unfertilized; 11 normal 
blastocysts partly or entirely bilaminar (fig. 5*, pl. 9; figs. 5 and 5A, 7 
and 7A, pl. 18; figs. 3, 8, and 8A, pl. 19). 
No. 347’. Jan. 30, 10:15 p.m. (interval 123 hours); 17 eggs: 4 un- 
fertilized; 13 bilaminar blastocysts measuring 1.1 to 1.24 mm. in 
alcohol (fig. 1, pl. 22). 
No. 349. Front foot wounded in trap; Jan. 30, 3:45 p.m., 5 eggs: 2 
unfertilized; one unilaminar blastocyst (fig. 4, pl. 8); 2 blastocysts 
with spreading entoderm (fig. 3*, pl. 8; fig. 12, pl. 17). 
JOURNAL OF MORPHOLOGY, VOL. 32, No. 1 
