560. Miss I. Sollas on Haddonella Topsenti. 
without any sharp limit into the granular pith which forms a 
thin layer. The inner surface of the pith is sharply defined : 
the pith is a hollow cylinder in which no cells of any kind 
can be distinguished. ‘The difficulty of pith-formation he 
overcomes by a theory: he asserts that a cap of spongoblasts 
over the tip of the fibre secretes spongin, thus adding to the 
length of the fibre. This is afterwards destroyed by cells, 
the process being comparable to the formation of bone-marrow 
by osteoclasts, and the active cells are consequently termed 
“ spongoclasts.”’ 
Minchin (4) relegates the whole matter of occurrence of 
cells in spongin-fibres to a footnote, saying that it is in need 
of re-investigation. 
My own observations are as follows :— 
The adult fibre shows, in cross or longitudinal section, 
three well-marked zones—(1) outermost, the cell-containing 
spongin-cortex, followed by (2) a layer of altered spongin- 
cortex, which surrounds (3) the pith: this may be solid, or 
having apparently yielded to tension may show a cleft-like 
cavity. (1) The cortex consists of cylindrical laminz of 
spongin, which are all concentred about the axis of the 
fibre, and of cells lying between the lamine. The youngest 
cells on the periphery of the fibre are of large size, in close 
contact on all sides with spongin. They are of the same 
colour as the spongin, but many shades darker ; the contrast 
which so pleased Flemming is absent here: the whole cortex 
is of the rich brown of spongin with a distinct tinge of violet 
superposed. The young cells are densely packed with 
spherical bodies ; the nucleus is central and lies in a small 
clear space, which gives the cell the appearance of being 
perforated (fig. 6). The outer surfaces of the cells are 
convex. Proceeding from the periphery of a section towards 
the interior, the cell-bodies become continually smaller and 
the spherules in them less numerous, consequently the cells 
in the inner layers are no longer in close contact with the 
spongin, but come to lie in cavities. In the innermost layers 
there seems to be little besides nucleus remaining in these 
cavities, which, however, are never quite empty. (2) The 
second zone might be called a transition-layer: it is 
more or less granular, at the same time somewhat fibrillated 
and contains cells, or at any rate nuclei (figs. 8 & 9). It is 
easy to convince oneself that it is formed by alteration of 
the innermost cortical layers, and that the nuclei are the nuclei 
of cortical cells. (8) The pith is granular, homogeneous, and 
destitute of cells. 
In the distal end of a growing fibre there are three regions 
