The Development of the Cartilaginous Skull ete. in Necturus. 397 
of the figure, the endoderm of the cleft still touches the ectoderm. 
On the right the endoderm, in the plane of section, is separate from 
the ectoderm allowing the mesectoderm of the mandibular arch to 
join that of the hyoid. The position of the mesothelial tissue in the 
mandibular and byoid arches is essentially the same as in fig. 1c, 
although, as the sections have become more ventral, the mesothelium 
of the mandibular arch has increased in quantity while that of the 
hyoid has slightly diminished. 
Between sections 1¢ and 1g, the mesectoderm cells of the hyoid 
arch have increased in number, as did the mesectoderm cells of the 
third arch between sections 14 and 1¢, moreover the mesectoderm 
of one side of the body has now united with that of the other, 
forming a bridge of mesectoderm below the floor of the branchial 
cavity. To explain more completely the relations in the posterior 
branchial region, figs. 1d, e and f are inserted between figs. 1c and 
1g. The sections follow one another dorso-ventrally in the al- 
phabetical order of the letters by which the figures are designated. 
In fig. 1d, the posterior part of the branchial cavity no longer 
falls in the plane of section and the walls of the third gill cleft 
are severed from their attachment to the floor of the branchial chamber. 
The median tissues of the third arch occupy relatively much the 
same position as in fig. 1c. In the fourth arch, the mesothelial tissue 
is united with the wall of the pericardium on the left, and is separated 
on the right merely by an intervening blood vessel. 
In fig. 1e, the severance of the walls of the third branchial 
cleft from the floor of the branchial chamber, begun in the plane 
of fig. 1d, is complete, and the mesectoderm of the third arch now 
meets that of the fourth arch. The mesothelial tissue of the fourth 
arch has united on either side with the wall of the pericardium. 
In fig. 1f, the mesectoderm of the third arch is not only united 
with that of the fourth arch, as in fig. 1e, but also with the correspond- 
ing mesectoderm of the opposite side of the head, forming a tissue 
that occupies the interval between the. anterior wall of the peri- 
eardium and the receding floor of the branchial chamber, save where 
the space is held by blood vessels. 
Returning to fig. 1g, one notices that the mesothelial tissue of 
the third arch, which was separate in fig. 1,f, has now in turn fused 
with the wall of the pericardium, while the separation of the walls 
of the second gill-clefts from the floor of the. branchial chamber 
allows the mesectoderm of the hyoid arch to unite with that of the 
