DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 377 
to that portion of Reichert’s membrane which covers the 
epiblast of the inner cell mass. 
According to both Selenka and Duval, the proliferation of 
the epiblast at the proximal pole of the ovum now proceeds 
rapidly, and the ovum assumes a more distinctly oval form (44, 
Taf. 1, figs. 6, 7, 9, and 10; and 9, pl. i, figs. 83 and 84). 
During this period (about the seventh day) Selenka figures 
a distinct line of demarcation between Rauber’s cells and the 
epiblast of the inner mass, but Duval draws no such distinc- 
tion ; on the contrary, he figures and describes the epiblast of 
the inner mass as continuous with the outer layer of cells. 
My specimens of these early stages of development of the 
ova of the mouse and the rat (fourth, fifth, sixth, and seventh 
days) do not confirm the interpretation which has been given 
by Selenka, Fraser, and Duval of the phenomena of these 
periods. . 
Towards the close of the period of segmentation, and before 
the blastodermic cavity appears, the ovum acquires an oval 
form (fig. 1, Pl. XXIII), which it retains until about the twelfth 
day of gestation. During the fifth day the long axis of the ovum 
changes its direction. On the fourth day (fig. 1, Pl. XXIII) and 
the commencement of the fifth day (figs. 2 and 3, Pl. XXIII) 
the long axis of the ovum runs from side to side; by the 
middle of the fifth day (fig. 4, Pl. I) it extends from roof to 
floor, or, according to Duval’s nomenclature, from proximal 
to distal end. I have been entirely unable to find a spherical 
blastodermic vesicle of the fifth day similar to those figured 
by Selenka and Duval. 
When the blastodermic cavity appears, its wall, over some- 
what less than half the extent of the ovum, is formed by a 
single layer of cells, and the remainder of the wall is consti- 
tuted by a mass of cells which is not separable into an inner 
mass of hypoblast and an outer layer ofepiblast. At this period 
(figs. 2 and 3, Pl. XXIII) the blastodermic vesicle lies free in 
the uterine cavity. Its thin and thick walls are directed 
towards the sides of the uterine cavity, and the extremities of 
its long axis towards the uterine margins. 
