52 MORGAN. [Vou. IX. 
to changes. In fact, in the living larva the openings of the 
gill-slits are sometimes distinctly seen separated from one 
another at the surface, and at other times are partially closed 
in by the surrounding walls. 
From the same series of sections another figure is drawn, 
taken nearer to the middle line; Fig. 69. Two gill-pouches 
are cut and a portion of a third. The two longer processes 
projecting into the oesophagus at ¢* and ¢? are the endodermal 
tongue-bars of the first and second gill-pouches. Into each of 
these the wall of the third body-cavity is protruding to line 
the interior of the tongue-bars with a layer of mesoderm. The 
extension of the body-cavity into the bars detween the gill- 
pouches has not as yet taken place. 
At this stage the dorso-anterior wall of the digestive tract 
begins to project forward to form the notochord. The mid- 
dorsal wall of the oesophagus beginning above the mouth and 
extending backwards beneath the collar constricts off partially 
from the more ventral portion of the tube, and it is the an- 
terior end of this constriction that pushes forward as a tube 
into the base of the proboscis. 
The third pair of body-cavities begins to fill up with fibre- 
like mesenchyme cells. The prolification takes place largely, 
perhaps entirely, from the somatic wall. As the walls of the 
body-cavities come in contact with the ectoderm, obliterating 
the blastocoel space, the loose mesenchyme cells are caught 
and pressed against the walls. They are conspicuous flattened 
cells, and show at first no signs of degeneration. At times 
I have thought they seemed to migrate on the outer side 
through the punctated layer into the ectoderm, but I have 
no good evidence to support such a proposition, so improbable 
from a priori considerations. The large collection of mesen- 
chyme cells at the base of the proboscis is very conspicuous in 
sections through that region. 
The next stage is inaugurated by the opening at the surface 
of the third pair of gill-slits. They open immediately behind 
the second pair, and although I have not here traced all of the 
intermediate stages, the method seems to be the same as in 
the first and second gill-slits. 
