392 ARTHUR ROBINSON. 
seventh day, a new cavity appears in the distal part of the 
trophoblast, either by extension from the cavity of the epi- 
blast, as in the mouse, or independently, as in the rat (7'C, 
fe, 12.) Pl XID). 
The Hypoblast.—The hypoblast forms the wall of the yolk- 
sac, which is gradually invaginated upon itself, chiefly by the 
growth of the trophoblast, but apparently not altogether so, for 
at the end of the seventh day (fig. 11, Pl. XXIII) the yolk-sac 
surrounds a greater portion of the trophoblast than it did at the 
middle of the same day (fig. 10, Pl. XXIII) ; and as the distal 
end (D) of the yolk-sac remains at a definite distance from the 
distal end of the uterine crypt, and as the invaginated proxi- 
mal end (P) of the sac is no nearer the distal at the end than 
at the middle of the seventh day, it is evident that during the 
latter portion of the seventh day there is no further extension 
distally of the trophoblast, consequently the yolk-sac must 
have extended towards the proximal end of the uterine crypt 
over the outer surface of the trophoblast, or there has been 
simultaneous growth of the distal trophoblast and the hypo- 
blast covering it. 
It is during the seventh day that the invagination of the 
yolk-sac is completed; and though the invaginated portion 
afterwards increases in length, it does so simultaneously with 
the growth of the epiblast and trophoblast which it encloses. 
After the completion of the invagination two areas of the 
hypoblast are clearly defined; (1) the invaginated or internal 
portion (H YJ), and (2) the non-invaginated or external por- 
tion (7 YE). The former lies in contact partly with the epi- 
blast and partly with the trophoblast; the latter is closely 
attached to the uterine mucosa. The only portion of the 
hypoblast which takes part in the formation of the embryo is 
the greater part of that which lies in contact with the 
epiblast, and it therefore is the embryonic hypoblast as dis- 
tinguished from the remainder, which is extra-embryonic hypo- 
blast. 
The Eighth and Ninth Days. 
In the early part of the eighth day the cavity in the distal 
