PROBOSCIS PORES ” IN CRANIATE VERTEBRATES. 548 
(PL 28, fig. 7). Eventually this depression becomes largo 
and deep, forming the preoral pit of the larva (PL 28, figs, 8, 
9, 10). The lining epithelium becomes modified into tho 
thick ciliated epithelium so conspicuous in later stages, and 
in the adult wheel-organ developed from it. For it is this 
preoral pit which is converted into the organ of Muller when 
the buccal cavity is formed at metamorphosis. Whether the 
thickened ciliated epithelium lining the preoral pit is actually 
derived from the ectoderm and not from the HatschelPs pit it 
is difficult to prove for certain, since the distinction between 
the mesoblastic cells and the ectoblastic cells at the mouth of 
the pit soon becomes indefinite. Moreover, there is an un- 
fortunate slight gap in my series between the oldest stage 
reared in the laboratory at Naples (51 hours) and the 
youngest free-swimming larva with two gill-slits I was able 
to obtain at Faro, and it is just at this stage that the 
proliferation of cells at this point begins. Nevertheless, the 
appearance in sections of these young larvae has convinced 
me that the lining of the preoral pit is indeed of purely ecto- 
dermal origin. How, then, can we account for the presence 
of the rod-bearing cells in the lining of Hatscliek’s pit itself? 
As mentioned above, they appear to be a specialised form of 
the slender cells composing the epithelium of the preoral pit 
(future wheel-organ). There can be hardly any doubt that 
the rod-bearing cells invade Hntschek’s pit from the outside, 
and are derived from the epithelium which grows in at the 
open mouth of the sac. In young larvae they do not yet 
occur among the lnrger mesoblastic cells; but in later stages 
they can be seen in increasing numbers, first near the opening, 
and then spreading over the inner surface of the sac. 
To sum up concerning the history of the ciliated wheel- 
organ of Muller and of HatschelFs pit in Amphioxus : The 
first pair of coelomic sacs or somites develop as outgrowths* 
which soon become nipped off from the anterior end of the 
archenteron. They are at first symmetrical, but soon the 
right enlarges to form the head-cavity, while the left, re- 
maining comparatively small and thick-walled, acquires an 
