xliv INTRODUCTION. 
all observers, that in the progressive growth of the ovum, 
the embryo of the Amphipoda is rolled within the egg in 
an opposite manner from that of the Isopoda. ‘The latter 
is folded backwards, so that the ventral appendages are 
developed on the external surface, whereas the Amphipoda 
is bent on itself, the ventral appendages being developed 
on the inner surface. Dr. Fritz Miiller states that, in 
Tanais, one of our aberrant genera, the development of 
the larva is after the manner of the Amphipoda, and not 
of the /sopoda, among which it is classified. 
The length of time between the epoch of the deposi- 
tion of the ovum in the ineubatory pouch, and the period 
of the emancipation of the young animal from the care of 
the parent, is probably about six weeks. We have ob- 
served that to be the time required in the genus Asellus, 
At first the egg is perfectly round. It shortly after- 
wards increases in one direction, becoming also somewhat 
larger in Amphipoda at one extremity. Indistinct seg- 
ments are now observable. The wall of the ovum is of 
an elastic character, and yields to the movement of the 
internal embryo. 
Probably about the middle of the period of incubation 
the embryo quits the egg, for we have constantly taken it 
from the pouch in a very immature condition, without 
being enclosed in the egg-case. The larva at this period 
is very immature, and enclosed within a general tunic, 
which, without having any apparent vital connection with 
the animal more than the original egg-case had, adapts 
itself in general form to the whole creature, and fulfils 
the duty of a protective tissue. As the embryo increases 
in dimensions and completeness of form, so the tunic cor- 
responds in size and form. At length, freeing itself from 
this case, the larva strengthens in its own development, 
but does not immediately quit the care of the parent. 
