DEVELOPMENT OF THE OPOSSUM (All 
opposite pole. In all of the eggs of litters 175’ and 347 (pl. 18) 
these facts are clearly shown. Blastocyst No. 175’ (9) is an 
extreme case in point (figs. 7 and 7A, pl. 19). In succeeding 
stages the embryonic area thickens progressively but slowly, 
until it reaches its maximum in blastocysts 1 to 1.5 mm. in 
diameter, after which it remains more or less constant until the 
embryo begins to differentiate. 
n. Cause of spreading of the entoderm 
In the spreading of the entoderm the chief factor is the active 
migration of the entodermal cells. The passive spreading, due 
to the enlargement of the vesicle, as had been suggested in the 
case of other mammalian vesicles, is, in the opossum, a negligible 
factor. An inspection of plate 18 will make this clear, for the 
vesicles are about as large when the entoderm begins to spread 
(fig. 1) as when it has reached the opposite pole (fig. 5). In 
fact, comparison of eggs of the same litter (figs. 5 and 7) show 
that the size of the vesicle bears no relation to the extent of the 
entoderm. My observations on numerous eggs at this stage 
(Nos. 347 and 299’) go to show that, when once begun, the 
spreading of the entoderm proceeds rapidly. I have not been 
able to demonstrate amoeboid movements in the cells, but 
processes sometimes occur on entodermal cells at about this 
stage (fig. 2, pl. 19). | 
0. Changing position of the vesicle in the egg 
In most of the eggs which mark the early stages in the spread- 
ing of the entoderm, the vesicle still occupies practically the 
center of the eggs as in previous stages (text figs. 1 and 2). In 
litter No. 352 (fig. 1, pl. 9) the tendency of the vesicle to ap- 
proach the shell membrane is already manifest to some extent; 
also in litter No. 175’ (fig. 7, pl. 19). In all later stages the 
vesicle occupies an eccentric position and in most cases it is in 
immediate contact with the shell membrane. This contact is 
established, therefore, for the first time about the beginning of 
the bilaminar stage, or about four days after the beginning of 
