OF THE PARASITIC ISOPOJDA. 
519 
been completely surrounded a second layer of cells has made its appearance under 
the central part of the blastoderm. These cells rapidly increase, and at the same 
time sink into, and become filled with the yolk. Meanwhile the blastoderm has 
surrounded the egg, the Keimstreif has been formed, and the involutions for the 
fore- and hind-guts have appeared. The hypoblast cells now begin to separate 
themselves again from the yolk, and gradually become arranged so as to form the 
wall of the liver, which, from the first, surrounds a considerable portion of the yolk. 
There is no yolk-membrane. The hind-gut grows forwards and becomes connected 
with the liver, in which the whole of the yolk is now contained, and eventually unites 
with the fore -gut. 
The following seems to me to be the most satisfactory explanation of the differences 
in the development of the hypoblastic organs in Cymotlioa and Oniscus. On com- 
paring the yolk-membrane of Cymotlioa with the mid -gut of Hessia (fig. 4, woodcut), it 
appears that a degeneration has taken place. In Hessia there is a distinctly cellular 
and functional mid-gut, while in Cymotlioa it is extremely thin and disappears 
entirely in the adult. In Oniscus it has completely disappeared, and at the same 
time the mode of development of the liver has become altered to enable it to perform 
to a certain extent the function of the yolk-membrane in enclosing the yolk. 
Instead of arising as in Cymotlioa (fig. 9) from a solid mass of hypoblast cells which 
penetrates into the yolk only to a small extent, and then gradually becomes hollowed, 
the liver is formed from hypoblast cells, which, at an earlier period, penetrate much 
more deeply into the yolk, and arrange themselves in such a way that, from the first, 
they surround a large portion of the yolk, and very soon completely enclose it. We 
must therefore regard the penetration of the hypoblast cells into the yolk in Oniscus 
as in no way connected with a primitive invagination or gastrula, but as a modification 
of the mode of formation of the liver and mid-gut found in Cymotlioa. 
In Astacus the case appears different, and until we have a more complete history of 
its development, probably no comparison of any real value can be made between it and 
Cymotlioa.. 
Fig. 1 9 is a transverse section through the thorax of an embryo, of the same stage 
as fig. 16. The hind-gut (HG) is a very narrow tube, with scarcely any lumen. On 
each side of it are the three liver coeca (L) ; their walls are very thin, the cells forming 
them scarcely seeming to be in contact. The muscles in the sides of the body and 
limbs are now distinctly striated. 
The dorsal organ (fig. 16) is still of the same form, and contains the same peculiarly 
shaped hollow. 
After their escape from the egg, the embryos remain in the brood-pouch of the 
mother until the whole of the yolk is absorbed. When they leave her they are very 
active, and swim well. Fig. 21 represents one of these young. It was one of a brood 
which I succeeded in bringing up by keeping the fish, in whose mouth the mother 
* Reichenbach, Zeit. fur Wiss. Zoo]., bcl. xxix., 1877 ; Bobretzicv, Astacus. 
MDCCCLXXVIII. 3 X 
