MATERIALS FOR A MONOGRABH OF THE ASCONS. 519 



trace of the gastral ray is to be seen. Fig, 50 on the same 

 plate shows almost exactly the same state of things, except 

 that the rays of the triradiate system have grown slightly, and 

 at their junction a minute gastral ray has appeared in the 

 gastral actinoblast, the nucleus of which is now no longer 

 placed over the centre of the spicule, but to one side. 



Fig. 51 shows a condition considerably in advance of the 

 foregoing ; the actinoblast is quite separate from the pore, ana 

 its nucleus has divided in two, while the ray secreted by it 

 has also grown considerably. In fig. 52 the condition is much 

 the same, except that the actinoblast still retains its connec- 

 tion with the pore. 



In Clathrina, sp. dub. (PI. 39, figs. 18 — 21), I was able to 

 observe some important stages in the formation of the fourth 

 ray, which place the origin of the actinoblast beyond all doubt. 

 The preparations were mounted in the usual way after removal 

 of the collar-cells as described above, so that it was possible 

 to make out clearly the actinoblasts of the basal system. Fig. 

 18 shows a triradiate system with its formative cells, and ex- 

 tending over it is a pore-cell, perfectly typical and normal 

 except in possessing two nuclei. In fig. 18a. the pore-cell is 

 drawn separately. One of the two nuclei is much deeper in 

 colour, and lies close to the minute gastral ray which has 

 already appeared at the junction of the rays of the triradiate 

 system. The pore-cell shows a commencing constriction, 

 dividing off the portion with the more deeply stained nucleus 

 as an actinoblast from the pore- cell proper with the paler 

 nucleus. 



Figs. 19 and 19a show a similar state of things, except that 

 the actinoblast has nearly left the pore-cell, being connected 

 with it by a drawn-out neck of protoplasm which, as can be 

 seen from its indented outline, extended between the collar- 

 cells. Finally, figs. 20 and 21 show the actinoblast completely 

 cut off and secreting the growing gastral ray. 



The stages figured in the two species enable us to construct 

 with certainty the following history for the first appearance of 

 the gastral ray. When a gastral ray is to be added to a tri- 



