21G 
THE VOYAGE OE H.M.S. CHALLENGER. 
The vesicular tissue of the ectochrote consists of a collenchjunatous matrix which 
stains with hsematoxylin, and in addition to the usual collencytes is crowded with empty 
oval cells, which give to it its vesicular character. These cells are about 0'02 mm. in 
diameter, and are apparently devoid of contents (PI. XXVII. fig. 14), except for an oval 
nucleus 0‘005 mm. in diameter, which bulges inwards from the side of the cell-wall. 
The cell can be traced from this stage backwards to a granular protoplasmic cell, not much 
larger than 0'005 mm. in diameter, which lies in an oval cavity now a little larger than 
itself. The cell increases in size ; the protoplasm becomes paler in colour, and from 
surrounding the nucleus as a granular heap, extends from it, by the time it has shifted 
its position to the side of the cell-wall, in fine, irregular, branching threads, which 
eventually altogether disappear. The fibres joining the sterrasters are clearly displayed ; 
they represent the fibrillated exterior of fusiform cells. In the associated inner fibrous 
layer of the cortex densely stained fusiform cells, 0'007 mm. in diameter, are to be met 
with ; in these the protoplasm is granular throughout and exhibits no trace of fibrillation. 
Choanosome . — The sarcenchyma of the mesoderm is of the usual character. The 
flagellated chambers are small, varying from 0'016 to 0’023 mm. in diameter, the aphodus 
and prosodus alike measure about O'OOS mm. in diameter (PI. XXVII. fig. 16). The 
fenestrse of the inner membrane are also very small, about 0 0 02 mm. in diameter. 
Spermatozoa (PI. XXVII. figs. 18-21). — In certain regions, near the main excurrent 
canals, the sarcenchyma becomes specially modified to give rise to sperm-clusters. The 
sarcencytes increase in size and take a more than usually deep stain with reagents, passing 
into large granular cells, of about 0*02 mm. in diameter, with well-developed nuclei and 
nucleoli. These next segment and give rise to oval clusters of small spherical cells, 
with a darkly stained marginal layer and spherical nucleus, separated by an intervening 
crescentic clear space. Clusters up to about 0’03 mm. in diameter completely fill the 
cavity which they occupy, but in subsequent stages they are separated from its walls by a 
considerable interval; thus in a cavity 0'05 mm., a size seldom exceeded, the cluster 
measures 0’044 mm. in diameter. The sarcencytes which form the wall of the vesicle 
are extended over its surface and take a somewhat deeper stain than those elsewhere. 
As the cluster increases, the component cells diminish in size ; in a cluster 0‘024 mm. 
in diameter they measure 0’004 mm., in one 0’043 mm. they measure 0‘003 mm. in 
diameter ; at the same time the outer marginal layer of each cell becomes thinner and 
paler. The increase in the number and consequent decrease in the size of the cells appears 
to take place from without inwards as regards the cluster, so that when the external cells 
are on the verge of maturity, a comparatively large granular protoplasmic cell may be 
seen in the centre of the group. Thus in the case of a cluster 0‘044 mm. filling a 
cavity 0'05 mm. in diameter, and composed of cells 0‘003 mm. in diameter, an oval 
granular cell measuring 0'012 by O’OOS mm., and containing an oval nucleus O'OOS by 
0'004 mm., persists in the middle of the group (PI. XXVII. fig. 19); it cannot be regarded 
