114 Victor E. Emmel. 
of the fourth stage limb shows a different condition. As may be 
seen in Figs. 1 and 2, there is strictly no free articulation evident, 
especially on the morphologically ventral side (see also Fig. 3, bk), 
where there is an almost perfect continuity of the exoskeleton from 
one segment to the other. On the dorsal side the union is less com- 
plete. A difference between the two sides of the breaking joint 
is evident even in the adult where the chitin is still much thicker 
on the ventral than on the dorsal surface. In section the region of 
the breaking joint can easily be identified by the characteristic color- 
ation which the chitin takes with Mallory’s connective tissue stain. 
With this reagent the exoskeleton, with the exception of the outer 
lamellee of chitin, takes on a bright blue color, whereas the line of 
fusion between the two segments is sharply differentiated by becom- 
ing dark red. 
The internal structure of the breaking plane of the limb may be 
described as follows. The epithelium (epidermis or “hypodermis’’) 
of the exoskeleton is continuous across the breaking plane, from one 
segment to the other. The connective tissue occupies the interior of 
the breaking joint and is complex in its arrangements. Certain 
structural relations of this tissue were discovered, which differ con- 
siderably from those previously recorded. They will presently be 
deseribed. 
Tt can be readily appreciated that the mechanism for preventing 
an excessive loss of blood is an important element in the perfection 
of a breaking joint adapted to the autotomous remoyal of the limb. 
Such a mechanism is supplied by the connective tissue structures. 
Frederieq (’92) was among the first to describe the separation of 
the blood eavities of the basipodite and ischiopodite by a “cloison 
membraneuse” or circular diaphragm, stretched across the distal part 
of the basipodite (p. 177). Similar descriptions have been written by 
Andrews (790) for Libina, and by Herrick (’98) and Reed (709) for 
the lobster and the crayfish. This transverse membrane was at once 
interpreted as a structure for preventing hemorrhage after autotomy. 
Andrews describes the membranous fold in the breaking joint of 
the spider crab as extending “from the epidermis to the central nerve 
and blood vessels,’ and states that after autotomy the membrane covers 
