84 Proceedings of the Koyal Society of Edinburgh. [Sess. 
there is no other opening (fig. 2). The two foramina lie very close 
together, and in Eu'pagurns are placed dorsally and internally on the 
diaphragm as seen after autotomy. Before describing the physiology of 
these structures a word must be said of the relative positions of nerve 
and vessel behind the diaphragm. The nerve is dorsal and internal to the 
vessel, and at the breaking plane lies almost upon the upper wall of the 
basi-ischium or second limb segment. A short distance proximal to its 
passage through the diaphragm the artery gives off about half a dozen 
small branches which run obliquely to the ventral wall of the limb, some 
passing through the epidermis to its outer side but running beneath the 
basement-membrane and the outer chitinous coating. Another set passes 
upwards, and is similarly distributed (fig. 3). 
Immediately after autotomy the crab extends the stump, and the 
diaphragm, now laid bare, faces dorsally. This condition lasts for forty- 
eight hours at most, and is caused by continued contraction of the extensor 
muscle of the basi-ischium. No blood is lost through the foramen, and this 
is probably accounted for by the fact that the part of the artery distal to 
the branches mentioned above is occluded. The diaphragm then bulges 
slightly, and in a period of weeks or days, according to circumstances 
mentioned below, a papilla or limb-bud grows out from it. 
The conditions immediately behind the diaphragm a few minutes after 
autotomy have been deduced from serial sections of the parts. The artery 
retracts and its torn end dilates, forming a little aneurism filled with 
plasma and lined with blood cells (fig. 3). The nerve also retracts slightly 
and the funnel- like flaps of the diaphragm are forced out over it owing to 
the great decrease of pressure on the outer side of the membrane. Meeting 
together, the flaps at once stop bleeding (fig. 4). 
The capillary branches running from the artery to the epidermis at 
once become dilated. Blood is extravasated from some into the dermis, 
and this, forcing the diaphragm outwards at various points, helps the 
action of the general venous pressure in closing the valve. A most im- 
portant point at this stage is seen in the extravasation of blood on the 
outer side of the epidermis. This causes the single layer of columnar cells 
to be detached at its ends and to curve inwards on the surface of the limb- 
stump beneath the diaphragm. The regenerative process now begins by 
the proliferation of cells of this layer from its free edges (fig. 5). 
2. Pa'pilla Formation . — By the fifth day a single sheet of cells has 
grown from the edges, and this approaches the foramen from all sides 
(fig. 6). Before this layer is complete, however, it is of great interest to 
find that provision is already being made for subsequent accidents by the 
