DEVELOPMENT OF THE OPOSSUM Mf 
3. Undivided, unfertilized uterine eggs showing little or no 
degeneration: 58, 173, 214, 287. 
4, Cleavage stages: 
a. From one to about four cells: 46, 52, 54, 203, 306, 293, 
81, 83, 299, 320. 
b. From about 8 to 16 cells: 85, 117’, 337, 342. 
5. Young unilaminar blastocysts: 
a. Containing from 25 to 35 cells, mostly without entodermal 
mother cells: 336, 173’, 191, 198, 203’, 314. 
b. Older stages up to 100 cells, mostly with entodermal 
mother cells: 50, 83, 298, 292, 88. 
6. Young blastocysts with distinct polar differentiation: 344, 
144’, 356, 349. 
7. Young blastocysts with spreading entoderm: 194’, 339, 352, 
43, 294, 175’, 347. 
8. The eerie stage: 347, 285, 299’, 205, 208, 290, 293, 
43, 306’, 352’, 82, 285’, 290’, 294! 189, 191’, 192, 343, 339), 94, 
55, 347, 353; 346, 360, 193’, 3437, 1897, 353! (few mesodenn 
cells). 
9. Primitive-streak stages: 353’, 346’, 320’, 192’, 193’, 205’, 
208’, 337’, 344’, 356’, 292’, 298’. 
10. Embryos: 349’, 336’, 314’, 342’, 360’, 321’. 
11. Unfertilized and degenerating eggs: 112, 415, 175, 194, 
anh) 282300 214", 318,303), 297,832, 7321". 
1. Securing the eggs. During the dolleenne season 1916 and 
1917 two stages were secured from each female after the method 
first employed by Bischoff on the rabbit. As the method has 
proved of great value to the writer in securing a complete series 
of stages, it is here discussed in some detail. 
The female is placed under anesthesia and one uterus is re- 
moved under aseptic conditions; the animal recovers and the 
eggs are allowed to ‘incubate’ in the remaining uterus for a calcu- 
lated period of time. In this way, by utilizing gradually ac- 
cumulating data on the rate of development, it became possible 
to secure almost any desired stage and thus fill in the gaps still 
appearing in the series. Thus, for example, I succeeded in 
securing from animal No. 353 eggs in which the mesoderm was 
