SENSE-ORGANS IN MEDUSAE 
759 
THE BEGINNING OF THE ADULT CONDITION 
With the appearance of the tentacles characteristic of the 
Chrysaora, or shortly after, the foundations of the other adult 
structures are established. The description of this initial stage 
will be taken from a specimen 10 mm. in diameter, in which the 
second set of tentacles reached to about the tips of the lappets. 
Although a good deal larger, the rhopalium and adjacent parts 
are about in the same proportion as in the preceding stage, com- 
pare figs. 26 and 33. 
The endoderm cells of the rhopalium are more narrowly colum- 
nar than in the last stage and the lumen of the canal reaches 
to the concretion-forming cells. The transition from the colum- 
nar cells to these is rather abrupt. These cells have increased in 
size and number and the whole mass of concretions is nearly 
spherical, fig. 33. 
The ciliated ectoderm of the rhopalium has increased still 
more in thickness and the nerve-fiber layer is still more marked 
than in the last stage. There has been but little change in the 
folds at the sides of the rhopalial ridge, yet it is in the ectoderm 
that the most important changes have taken place. In the first 
place, the epithelium of the lower surface of the rhopalial ridge, 
which was slightly thickened in the pelagia stage, is now very 
much more thickened, the cells becoming cuboidal or almost 
columnar, while the epithelium of the roof of the sensory niche 
adjoining the rhopalial ridge has suffered a similar change. 
But the chief step in advance is the formation of the ''olfactory 
groove," or, as von Lendenfeld prefers to call it, ''the dorsal sen- 
sory groove." This is a shallow saucer-shaped depression in the 
exumbral surface of the hood just above the base of the rho- 
palium, figs. 33 and 34, s.g. In this groove the ectoderm is consider- 
ably thickened, being composed of a single layer of columnar 
cells. These cells are deepest in the deepest part of the groove. 
In specimens of about the same size as the one just described but 
in which the evaginations that are to form the second set of ten- 
tacles are not longer than they are broad, a shallow dorsal groove 
occurs, but it is clothed simply with the ordinary flat epithelium. 
