542 
EDWIN S. GOODRICH. 
was disproved by MacBride ( 11 ), whose results were accepted 
by vail Wijhe (14), and since Legros has recognised his 
mistake the controversy may be dropped. As, however, it 
seemed desirable to reinvestigate the whole question, an 
account is here given of the development of the pit and 
Muller’s ciliated organ from their first appearance to the 
adult condition. 
The first pair of coelomic sacs are given off as lateral' 
pouches at the extreme anterior end of the archenteron, and 
can be seen in embryos about twenty-four hours old still in 
this condition (PI. 28, fig. 3). Later the pouches become 
nipped off, and come to lie symmetrically on either side of the 
notochord (PI. 28, fig. 2). Even in these very early stages 
the left may have a rather thicker wall than the right sac 
(PI. 28, fig. 1). At about the 30-ho.ur stage the right sac 
begins to expand, it walls thin out, and later on it expands to 
form the head-cavity of the larva. The left sac with its 
thicker wall at first remains spherical, but later becomes 
flattened, and takes up a position lying transversely between 
the notochord above and a backward prolongation of the 
right head-cavity below (PI. 28, fig. 5). Its outer end now 
becomes applied to the ectoderm on the left side. Larvae 
about fifty hours old show that an opening has been pierced 
at this point of contact, placing the coelom in communication 
with the exterior. At the same time the large cubical cells 
lining the cavity acquire cilia (PI. 28, fig. 6). In larvae with 
two gill-slits the left coelomic sac has enlarged, spreading 
further towards the right side, while the ectoderm round the 
opening has grown inwards, tending to form a depression 
somite,, as described by Legros (10) ; nor, again, from the remains of 
the communication of the coelomic pouch with the gut, as alleged by 
MacBride (11), but from a little group of cells appearing quite early 
just above the mouth. Several years ago I traced these cells to a stage 
about thirty hours old, before the opening of the mouth ; but since I 
was unable to find out their first origin, I refrained from publishing 
my results. They are the cells figured recently by Smith and Newth 
(PI. 18, fig. 4), who are indeed correct in their surmise that they may 
represent the rudiment of the nephridium (13). 
