426 DAVENPORT HOOKER 
curred, if primary fusion of the wound surfaces had not taken 
place. Primary fusion of the severed parts of the embryo always 
ensued when the wound surfaces were carefully apposed, but 
such embryos afford little or no evidence of the individual 
processes leading to the healing of the wound, because no active 
regenerative changes are visible. Some evidence on this point 
is afforded, however, by those embryos in which the continuity 
of the cord was interrupted at either or both ends after the epi- 
dermis had regenerated sufficiently to cover the cuts and hold the 
reversed piece in position. We shall consequently examine 
these latter embryos rather in detail. 
A period of primary repair follows immediately on the opera- 
tion, whether the cord be severed in one place or in two, even 
though in the latter case the reversal end-for-end of the tissue 
between the cuts accompanies the operation. The epidermis 
plays the major part in this repair, covering over the wound 
surface so completely that it pushes down between the cut cord 
ends if they are not fused. The mesenchyme proliferates rap- 
idly and fills up the spaces made by the operation. The open 
ends of the neural tube become closed by a shifting of the cells 
already present and new composite myotomes are formed by the 
fusion of their dorsal and ventral halves. 
Sections show that the cephalic cut in nearly all cases passes 
through the caudal extremity of the medulla. The second cut is 
found from 1 to 1.5 mm. caudad to it. The caudal (originally 
cephalic) end of the reversed portion of the cord has usually 
somewhat greater diameters than the cephalic (originally caudal) 
end, due to the small part of the medulla attached to it. In 
many embryos the cut ends of the cord do not lie directly oppo- 
site one another, but deviate in various directions from true align- 
ment. The piece reversed is so small that it is difficult to place 
it exactly in position, but such deviation, unless excessive, does 
not necessarily hinder the ultimate reunion of the cord. 
During this period of primary repair the embryo as a whole 
continues to grow and the characteristic feature of all frog 
embryos in which a portion of the dorsum has been reversed 
begins to appear. This is the peculiar hump on the back which, 
