Miss I. Sollas on Haddonella Topsenti. 561 
distinguishable, succeeding one another along its length. At 
the extreme apex is a region where the pith lies naked in the 
cap of spongoblasts. That Polejaeff overlooked this may be 
due either to the fact that the fibres he examined had ceased 
to grow, or it may be explained by irregularity of form or 
change of direction of the fibre. In the present material it is 
easy to obtain sections of an apparent apex (fig. 7), while the 
true growing point lies bent to one side and would appear 
many sections further on. I cannot say whether the same 
would be true of Lanthella. 
In the second region, which is of varying length, the pith 
is covered with a single layer of spongin without cells em- 
bedded in it. This passes, by no means abruptly, into the 
cell-bearing region, which includes the whole of the older 
parts of the fibre. At first the cells are very sparsely 
scattered (figs. 4 & 7) and may be found outside the first- 
deposited layer of spongin, then included between the first 
two layers, with a second set of cells on the outside, and so on 
—always with cells on the outside as long as the fibre is still 
growing in thickness; when this growth has ceased the 
outermost layer is of spongin. From the rarity with which 
one finds cells on the outside of a fibre quite uncovered by 
spongin, one concludes that the deposition of this substance 
must take place rapidly. 
The fibre-tips which are going to form adhesive disks differ 
from the ordinary tips in the great broadening of the pith at 
the apex and in the fact that the pith is quite without cell- 
covering over its flat distal surface. Accordingly, in the fully 
formed disk, the spongin which closes in the pith-cylinder at 
the extremity shows manifest signs of having arisen from 
secondary ingrowths from the spongin-cortex (Pl. XXIX. 
fig. 10). 
‘he spongoblasts are many layers deep (PI. XXVIII. 
fig. 5). Lower down on the fibre the layers are fewer and 
the cell-bodies larger, but still very much smaller than those 
of the cells applied to or included in the fibres and charged with 
spherules, between which cells and the ordinary spongoblasts 
outside the fibre I have looked for transition-forms in vain. 
In material stained in bulk with Ehrlich’s hematoxylin 
and afterwards well washed out with acid alcohol the spongin 
did not stain and lost its violet tinge, while both pith and 
transition-layer were well stained, particularly the latter. 
In sections stained with iron hematoxylin (Heidenhain) 
the whole fibre was stained, and the nuclei in the transition- 
layer show up well after this treatment. 
