EMBRYO AND EMBRYONIC MEMBRANES 217 
The splitting of the mesoblast is never complete; but on the 
contrary the undivided margin begins to thicken after the fourth 
day, and gradually forms a ring of connective tissue that surrounds 
the umbilicus of the yolk-sac (Figs. 128 and 129). When this 
ring closes, about the seventeenth day, it forms a mass of con- 
nective tissue uniting the yolk-sac and albumen-sac. (See 
below). 
During the first few days of incubation the albumen loses 
water rapidly, and becomes more viscid, settling, as this takes 
place, towards the yolk-sac umbilicus. Thus the amniotic sac 
containing the embryo lies above; beneath the amniotic sac comes 
the yolk, and the main mass of the albumen lies towards the 
caudal end of the embryo (Figs. 128 and 129). 
The allantois expands very rapidly in the extra-embryonic 
body-cavity, and the latter extends by splitting of the mesoblast 
into the neighborhood of the yolk-sac umbilicus. When the 
allantois in its expansion approaches the lower pole of the egg, 
it begins to wrap itself around the viscid mass of the albumen 
accumulated there. In so doing, it carries with it a fold of the 
chorion, as it must do in the nature of the case, and thus the 
albumen mass begins to be surrounded by folds of the allantois 
with an intervening layer of the duplicated chorion. These 
relations will be readily understood by an examination of the 
accompanying diagrams (Figs. 128 and 129). In this way an 
albumen-sac, which rapidly becomes closed, is established out- 
side of the yolk-sac, and the two are united by the undivided 
portion of the mesoblast around the yolk-sac umbilicus. This 
connection is never severed, and in consequence the remains of 
the albumen-sac is drawn with the yolk-sac into the body-cavity 
towards the end of incubation. 
The sero-amniotic connection, which persists throughout incu- 
bation, has an important effect on the general disposition of the 
embryonic membranes. It is formed, as we have seen, in the 
closure of the amnion, by the thickened ectoderm of the suture; 
this ectodermal connection is, however, absorbed and replaced 
on the fifth to the seventh days by a broad mesodermal fu- 
sion, which maintains a permanent connection between amnion 
and chorion. One important result of this relation is that the 
albumen-sac, which is formed by the duplication of the chorion, 
is prolonged by a tubular diverticulum to the sero-amniotie 
