516 
MR, J. F. BULLAR ON THE DEVELOPMENT 
the point of junction of the fore- and hind-guts. Fig. 18 shows the yolk, liver coeea, 
and fore-gut teased out in the fresh state ; the hind-gut, owing to its extreme ten- 
derness, has been broken off. The yolk-membrane is very thin, and bursts so as to let 
the yolk rush out on very slight pressure. Its presence is most easily demonstrated 
by cutting off the abdomen close to its junction with the thorax, and then allowing 
the cover-glass to press very gently on the embryo. This causes the yolk to protrude 
from the cut end, and its outline may be seen to be quite smooth and sharp; the least 
extra pressure at once causes the membrane to burst, and the yolk then immediately 
streams out. With care, the whole yolk-sac may be dissected out still attached to the 
fore-gut, as in fig. 18. 
In the process of hardening, the yolk-membrane always breaks, and thus allows the 
yolk to escape into the body cavity, and to occupy positions in which it is not found 
in the living animal. For this reason sections are not entirely trustworthy, the hind- 
gut and liver coeca often appearing embedded in the yolk, whereas in the living 
embryo they are quite free from it. The yolk-membrane cannot in any one section 
be traced the whole way round the yolk ; but in those places where it is supported, 
such as on each side of the nerve-cord and between the liver cceca, the membrane 
can be seen in almost every section. There seem to be elongated nuclei here and 
there in it. 
I think there can be no doubt that the yolk-membrane and the cells forming the 
liver are to be regarded as the hypoblast or lining of the mid-gut, for the liver is only 
a diverticulum from that organ. 
It appeared in an earlier stage (stage III.) that the liver arose from a mass of cells 
on each side of the embryo, and apparently had no connexion with the rest of the 
digestive system. 
This mode of development is perfectly explained if the yolk-membrane is regarded 
as the mid-gut. Though its presence cannot be demonstrated in the earlier stages, it 
no doubt really exists in them ; and it would probably not be wrong to trace it back 
to some of the lower layer cells in the second stage, from which the liver (stage III.) 
arose, or even perhaps to the lower cells in the first stage. It is thus probable that 
the liver does not arise, as at first sight might appear, from two isolated masses of cells, 
but in the normal way by a prolongation of the cavity of the mid-gut. 
The arrangement of the parts becomes clear on remembering the large quantity of 
food-yolk in the egg/" It may be supposed that, owing to the growth of the hind-gut 
being more rapid than the absorption of the food-yolk, the point of junction of the 
hind- and mid-guts gets carried more and more forward (see diagrams A, B, C, fig. 3, 
woodcut) along the dorsal surface of the yolk until at last it reaches the fore-gut, into 
which the hind-gut opens, the mid-gut remaining as a sac enclosing the yolk, and 
opening at the junction of the fore- and hind-guts. 
As the yolk disappears the mid-gut disappears with it, and the alimentary canal 
* More than in any other Crustacean except Astacus (?). 
