DEVELOPMENT OF THE OPOSSUM Hae 
clear-cut nor as definitely and neatly circular as in later stages. 
In surface view of preparations the junctional line between the 
two areas can always be made out (figs. 11 and 12, p'. 19); but 
in sections some difficulty is experienced in trying to determine 
with exactness the marginal cells of either area, for the cells of 
the embryonic area have not yet assumed that density of pro- 
toplasm characteristic of later stages. The embryonic ectoderm 
is at first comparatively thin, consisting of a single layer of 
somewhat flattened cells as in figures 5 to 7, plate 18, and 3 
and 8 to 10, plate 19. The area gradually thickens and the 
cells become cubical in section (fig. 4, pl. 10, fig. 4, pl. 19). In 
the younger eggs the nuclei are, therefore, further apart, but, 
as they multiply, they become more and more crowded until 
they are almost or quite in contact (compare fig. 3B, pl. 20, and 
fig. 12, pl. 19). Im surface views the area is studded with 
mitotic figures (fig. 3B, pl. 20). 
The surface views presented in figures 3B, plate 20, and 12, 
plate. 19, of which the latter is the more advanced, show that 
the junctional line between the embryonic and the trophoblastic 
areas is quite definite, sometimes being marked by a con- 
tinuous sharp line around the entire area. Figure 3B, plate 20, 
is especially instructive in this connection, since it represents 
in surface view (x 500) a portion of the same area shown in 
figure 3A in section (X 200); it is the portion removed before 
imbedding from point A, figure 3. There is, neither in the 
section nor in the surface view, any great dfference of tone 
between the two areas, but the junctional line (XX) or margin 
of the embryonic area may easily be located where the ecto- 
dermal nuclei become less crowded. The junctional line is 
always more clearly defined in specimens fixed with a fluid that 
will bring out the cell membranes, as in figure 12, plate 19. It 
is seen that the line is formed of the cell membranes of con- 
tiguous cells bordering the areas. It is a perfectly definite 
structure: the marginal cells of the two areas do not intermingle 
nor do transitional cells occur between the two types. 
