87 
1914-15.] Eegeneration of the Legs of Decapods. 
examination shows that the following changes are taking place ; — At 
certain points an invagination of epidermis appears. The outer cells 
continue to secrete chitin, and so a plate is formed down the middle of 
the invagination. This splits and a constriction appears on the outer side. 
At the extreme tip of the invagination a double row of cells can often be 
seen pushing into the muscle-masses. These cells also form a chitinous 
plate between them, and so the large tendon is formed. This plate is 
attached at the point of constriction, and in the fully formed leg comes 
to be placed at the end of the synovial membrane of the joint, into which 
the constricted part of the papilla develops. 
Thus the papilla increases in length, and at the full period of its growth 
(which varies with the age of the crab) it is only about one-fifth of the 
size it assumes immediately after moulting takes place. All the parts are 
laid down compactly, and the epidermis is several cells thick. The muscle- 
masses have developed into fibres, and the nerve is connected with each 
one of these. A few words may be said here on the muscle-differentiation, 
but these must necessarily be brief, as the subject is one which will receive 
special treatment in a later publication. 
The cell-nests mentioned above come together and form a plasmodial 
mass. The large nuclei then form up into parallel lines with cytoplasm 
between each pair. Very soon this material shows longitudinal striation, 
and later it takes the appearance of a bundle of hyaline tubes running 
between rows of nuclei (fig. 10). In mature papillm these tubes show cross 
striation, and a Dobie’s line is visible between the dim bands (fig. 11). 
They may now be identified with the fibrillse of the adult muscle. In 
transverse section a ring of nuclei is seen to enclose the bundle of fibrillse, 
but the outstanding difference between this young muscle fibre and the 
normal one is the absence of interstitial substance or sarcoplasm. A net- 
work of connective tissue fits loosely round the whole, and this represents the 
sarcolemma. Its origin can also be traced to the epidermis (fig. 12). 
To summarise, the appearances of papillae immediately before moulting 
are as follows : — 
In the lobster papilla, which may be regarded as the least perfected 
one, the miniature limb is straight and is almost a proportionate 
model of the normal appendage. It has no functional activity, 
no calcareous coating like the rest of the shell, and is covered 
by a thin layer of chitin. On section it shows an epidermis 
beneath the chitin coat, composed of several layers of cells. 
Nerve and vessel are present, and these pass to the muscle- 
masses. Muscle fibril! 96 are striated and enclosed in bundles 
