DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 9391 
however, I have not been able to find two similar cavities, 
neither apparently has Duval; and as the appearance of some 
of my sections corresponds very closely as regards this point 
with the figures given by Selenka in the first part of his 
studies (44), I am inclined to the belief that the cavity of the 
trophoblast in the mouse does not arise independently, but is 
the result of the extension of the cavity of the epiblast into 
the trophoblastic mass. 
Summary of the Events of the Seventh Day. 
The Epiblast.—The epiblast is projected by the increase 
of the trophoblast against the proximal wall of the yolk-sac 
which is gradually invaginated, and thus at the commencement 
of the eighth day the epiblast lies at the distal end of a long 
cavity, which is bounded by hypoblast (fig. 11, Pl. XXIII). 
Whilst this alteration of position is taking place the epiblast 
increases in amount, changes from an oval (fig. 9, Pl. XXIII) 
to amore circular form (fig. 10, Pl. XXIII), and a cavity 
develops in its interior (figs. 11 and 12, Pl. XXIII). This 
cavity is afterwards extended and becomes the cavity of the 
amniotic sac, from which the central canal of the nervous 
system is separated by the closure of the medullary folds. 
The Trophoblast.—This portion of the ectoderm pro- 
liferates more rapidly than the epiblast, and the rapid growth 
of its central portion is the main cause of the invagination of 
the yolk-sac, its length increasing from 22 wp (fig. 9) to 106 u 
(fig. 10). The increase of the margin of the trophoblast and 
its extension along the sides of the uterine crypt result in the 
appearance of a cavity apparently within the trophoblast 
- (T'C., fig. 10), the primary cavity of the trophoblast. This 
cavity is entirely obliterated by the approximation and fusion 
of its lateral boundaries, after which the trophoblast has, for a 
time, the appearance of a solid rod, which lies partially within 
and partially outside the cavity formed by the invagination of 
the yolk-sac (fig. 11), and for the convenience of description 
in the later stages these two portions will be respectively the 
distal and the proximal. Afterwards, towards the end of the 
