CHESTER. — STIUCTUKE OF PSKUDOPLEXAURA CRASSA. 765 



cells of the axis epitheliuni over a mass of clesmocytes, and 1 can 

 account for tlie possible enclosure of spicules in an axis in the same 

 abnormal manner. 



A. Schneider found that in Eunicella the axis of the branch (Nebe- 

 naxis) is at first separated from that of the stem, with which, however, 

 it is later united. It is dillicult to see how such an axis can be ex- 

 plained as the result of the secreting activity of an ectodermal epi- 

 theliimi, except in cases in which the branch is secondarily united 

 (by anastomosis) to a stem, as in fan corals; but Eunicella does not 

 usually have such a secondary union of branch and stem. Perhaps, 

 however, the conditions in Eimicella are not essentially different from 

 those which are met with in Pseudoplexaura, where I find that a sharp 

 demarcation line between the axis of the branch and that of the stem 

 also occurs (Plate 4, Fig. 61) ; here it is due to the fact that all branches 

 of the axis are adventitious in respect to the marrow. At the region 

 of branching, the marrow of the stem-axis is separated from that of 

 the branch by the secreted cortex of the stem-axis. The walls of the 

 marrow chambers in the branch were therefore formed after the axis- 

 cortex of the main stem possessed an appreciable thickness (Fig. 61, 

 ctx. ax.). But the existence of a stem-cortex between the marrow 

 chambers of the stem and those of the branch is not inconsistent with 

 an ectodermal origin of the epithelium secreting the axis of the branch, 

 because axis-cortex is formed in the same manner as axis-marrow. In 

 both cases the horny matter is laid down in the form of walls of 

 chambers; and these differ only in the size and shape of the cavity 

 and in the thickness of the wall. The chambers of the axis-cortex are 

 smaller than those of the marrow, nevertheless they vary greatly 

 among themselves in size (Fig. 57, c^.r. ax.). It is assumable that, after 

 some of these axis-cortex chambers of the stem had been formed 

 (Fig. 61, ctx. ax.), other chambers with the characteristically thinner 

 walls and larger cavities of the marrow, may have arisen at the place 

 where a branch was about to be produced. The walls of these cham- 

 bers would, then, be secreted by the same epithelium that recently 

 had been building smaller chambers as an axis-cortex of the stem. 

 The epithelial patch at the distal end of the axis of the branch would 

 be composed of cells which had changed somewhat the character of 

 their secretions, so that henceforth they would produce the larger 

 thin-walled chambers characteristic of the marrow, whereas the 

 remaining cells (at first situated in the periphery of this terminal 

 patch) would continue to produce the smaller chambers, with thicker 

 walls, such as they had been producing as axis-cortex of the stem; 

 but now as the axis-cortex of the branch. 



