DEVELOPMENT OF -GERMINAL LAYERS IN MAMMALS. 3885 
complete; but a full description of the events of the seventh 
day will be more rapidly given and more easily understood 
after a consideration of the sections represented in figs. 9, 10, 
11, and 12, Pl. XXIII. 
Fig. 9, Pl. XXIII, is the eighth of a series of sixteen longitu- 
dinal sections of a mouse ovum, cut in vertical transverse sec- 
tions of the uterus in the early part of the seventh day. The 
sections are slightly oblique, and the distal end of the yolk-sac 
is somewhat curved, so that the whole length of the ovum, 
232 u, does not appear in any one section. 
As compared with the ovum represented in fig. 8 the yolk- 
sac 1s longer, but not so wide; indeed, in the distal portion of 
the sac the walls are in contact (fig. 9). 
The walls of the yolk-sac consist of nucleated protoplasm, 
which stains but faintly with carmine. The protoplasm is most 
abundant and the nuclei are most numerous in the proximal 
wall of the sac, which is partially invaginated by the epiblast 
(#), and from the proximal wall a few protoplasmic strands 
extend to the lateral walls of the cavity. The lateral walls of 
the yolk-sac are thin, except in the distal portion (fig. 9), where 
the thickness of the walls is more regular and the nuclei are 
placed at more uniform distances. 
The epiblast (4) is an oval mass of protoplasm, 61 « long 
and 28 broad. It contains many nuclei, which are very 
irregular in size. It is entirely surrounded in its distal half 
by hypoblast (HY), and in its proximal half by trophoblast 
(7), and it is not separable into distinct cell territories. 
The trophoblast (7) forms the proximal end of the ovum; 
it is 22 thick. It covers the proximal end of the epiblast, 
and, extending round it slightly, overlaps the proximal part of 
the hypoblast (see the left side of fig. 9). Like the epi- 
blast, the trophoblast is not divided into cells. Its nuclei 
are not so large or so irregular in size as those of the epiblast. 
Both the epiblast and the trophoblast stain more deeply with 
carmine than the hypoblast. 
Fig. 10, Pl. XXIII, is a pepe seccmebn of an oblique longi- 
tudinal section of a mouse ovum at about the middle of the 
