Points tn the Histology of Cclenterates. 261 
subsequent paper). I therefore believe that, starting from 
the cavity of the capsule, which has previously been formed 
with the mner wall of the subsequent perfect cyst, the deve- 
lopment of the thread proceeds in the protoplasm of the cnido- 
blast, and that after completion the thread is introverted, 
commencing with the tip and ending with the thickened basal 
portion, so that this enters the capsule last. The develop- 
ment of the outer wall of the capsule is the last to take place, 
and, as it appears to me, does not occur until the spot is 
reached at which the cyst comes into operation. The 
extremely interesting formation of the thread, from its histo- 
logical aspect, will be described in my subsequent paper. 
The lamellar arrangement in the protoplasm of the cells of 
the nettle-pad, as described by the brothers Hertwig *, and 
as is actually seen in animals macerated in a mixture of osmic 
and acetic acid, is due to the disposition of the thread around 
the wall of the nematocyst. I did not clearly grasp this 
point until I examined the pad, both in its living state and 
when treated with 50 per cent. acetic acid; the latter reagent 
causes the thread to become sharply defined, though in the 
living object it is only to be detected with difficulty (as the 
tissne dies away it becomes more and more distinct), The 
young cells pass from the pad to the filament in Yorskalea, 
but to the tentacle in Carmarina. ‘The same is doubtless 
true for the cnidoblasts of the peronia of the Narcomedusz 
and of the mantle-rivets which are found in the Geryonide. 
Thus the presence of mantle-rivets on the sensory bodies also 
probably points to the fact that the latter represent rudimen- 
tary tentacles. 
In conclusion, I would just briefly mention that in the 
investigation of Aleyontum acaule I arrived at definite views 
with regard to the formation of the spicula. In the ecto- 
derm of this form cells occur to which the term indifferent 
may be applied; these coalesce here and there into groups, 
and by fusion give rise to structures which are to be con- 
sidered as matrix-elements of the spicula. They furnish the 
form of the future spiculum, and then secrete within them- 
selves the calcareous substance, in which the nuclei are at 
first still distinguishable, but which finally so completely fills 
the whole that nothing more is to be seen of the organic base 
and the structure appears homogeneous and shining. ‘This 
transformation is accomplished in the mesoderm. 
* O. and R. Hertwig, loc. cit. 
Ann. & Mag: N. Hist. Ser. 6. Vol. ix. 19 
