556 
H. V. WILSON 
ous pore membrane finally begins to extend across the pore. A 
single nucleus not differing in appearance from other epidermal 
nuclei is associated with the closure of a pore. It lies at the mar- 
gin round which it is shifted back and forth, in the first stage of 
closure, probably by wave-like movements of the protoplasm 
similar in some respects to those occurring in plants cells (Nitella, 
e.g.). These movements of the nucleus are quick and easily 
observed. For instance a nucleus made the complete circuit of 
a widely open pore in about one minute. After closure of the 
pore, other nuclei beside the pore nucleus may pass into the area 
which covers the pore canal (fig. 11). The constant presence of 
a nucleus at the pore and its quick changes of position strongly 
suggest that it is in some way physiologically concerned in pore 
closure. 
The extension of the epidermis to form a pore membrane does 
not necessarily involve the rest of the dermal membrane, and hence 
after the epidermis has spread across the pore, the pore canal 
may still ren ain open, in which case its outline is distinguishable 
on focussing below the surface. Usually after closure of the pore, 
the outline of the canal is suddenly lost to view. This must be 
due to centripetal streaming of the mesenchym.e of the dermal 
membrane, induced by some local contraction in the epithelial 
wall of the canal. Perhaps in nature the pores commonly remain 
in this condition until they reopen. At any rate this is the state 
in which closed pores are usually found in preserved material 
(fig. 11). In excised pieces of sponge kept under a cover glass 
the pore canal may completely or almost completely disappear. 
In this latter case the area of the canal diminishes greatly in size 
and its outline becomes rough and vague (figs. 13 and 14). This 
small and vaguely outlined area represents the central region of a 
pore membrane, and indicates how far the mesenchyme of the 
dermal membrane has streamed inwards in its obliteration of the 
pore canal. 
The fonnation of a pore membrane is a contraction phenomi-enon 
which involves only the epidermis. The closure of the pore canal 
is probably also primarily a contraction phenomenon, which in this 
case involves the epithelial lining of the pore canal. The epithe- 
