264 
wards; the mouth is situated between the anterior pair. 
The whole embryo covers but about a third of that portion 
of the yolk in sight. At this time the imner egg membrane 
(blastoderm-skin ?) was first detected. 
The outer membrane, or chorion, is structureless; when 
ruptured the torn edges show that it is composed of five or 
six layers of a structureless membrane, varying in thickness. 
The inner egg membrane is free from the chorion, though it 
is in contact with it. Seen in profile it consists of minute cells 
which project out, so that the surface appears to be finely 
granulated. But on a vertical view it is composed of irregu- 
larly hexagonal cells, sometimes 5-sided, and rarely 4-sided, 
hardly two cells being alike. The walls of the cells appear 
double, and are either strongly waved, or have from three 
to five long slender projections, with the ends sometimes 
knobbed, directed inwards. These cells are either packed 
closely together, or separated by quite a wide interspace. 
In a subsequent stage the oval body of the embryo has 
increased in size. The segments of the cephalothorax are 
indicated, and the legs have grown in length, and are doubled 
on themselves. But the most important change is in the 
small size of the rudiments of the mandibles, compared with 
the remaining five pairs of limbs; and the origin of two pairs 
of yills, forming pale oblique bands between the sixth pair of 
legs and the end of the abdomen, which forms a narrow 
semicircular area. 
A later stage is signalised by the more highly developed 
dorsal portion of the embryo, and the increase in size of the 
abdomen and the appearance of nine distinct abdominal seg- 
ments. The segments of the cephalothorax are now very 
clearly defined, as also the division between the cephalothorax 
and abdomen, the latter being now nearly as broad as the 
cephalothorax, the sides of which are not spread out as in a 
later stage. At this stage the egg-shell has burst, and the 
“amnion” increased in size several times exceeding its 
original bulk, and has admitted a corresponding amount of 
sea water, in which the embryo revolves. At a little later 
period the embryo throws off an embryonal skin, the thin 
pellicle floating about in the egg. 
Still later in the life of the embryo the claws are developed, 
an additional rudimentary gill appears, and the abdomen 
grows broader and larger, with the segments more distinct ; 
the heart also appears, being a pale streak along the middle 
of the back extending from the front edge of the cephalo- 
thorax to the base of the abdomen. 
Just before hatching the cephalothorax spreads out, the 
