404 Julia B. Platt 
derm and endoderm. The T-shaped oral fusion at »n« lies in the 
space »m« of fig. 2, and is bounded posteriorly by the transverse 
mandibular bridge of mesectoderm. ‘The successive branchial pockets 
extend at their lateral margins both higher and lower than the 
dorsal and ventral walls of the branchial chamber from which they 
arise. The hyomandibular cleft is exceptional in ending ventrally 
in the lateral wall of a median ventral outgrowth from the floor of 
the branchial cavity. 
This outgrowth, the thyroid Anlage, sectioned in fig. 1 A, lies 
between the transverse mesectodermic bridges of fig. 2. The hyoid 
bridge is dorsal to the outgrowth, and the mandibular ventral, but 
between the thyroid outgrowth and the mid-ventral part of the man- 
dibular mesectoderm lies the mesothelial tissue of the united hyoid 
and mandibular arches, as seen in fig. 4, which represents a cross 
section through the thyroid outgrowth in an embryo slightly younger 
than that modeled in fig. 2. As fig. 4 is drawn with a higher magni- 
fication than that used in figs. 1 a—12, the tissues are less diagram- 
matically represented. The nuclei of both mesoderm and mesectoderm 
are outlined, while the yolk granules are represented by dots. The 
section is bounded above by the anterior wall of the hyobranchial 
cleft. Immediately below this endodermie wall lie mesectoderm cells 
of the hyoid arch. Those of one half of the arch have just come 
in contact with those of.the opposite half, and as yet the uniting 
bridge is but one cell deep. The centre of the figure is- occupied 
by the thyroid outgrowth, below which the section passes through 
a wide pocket of mesothelial tissue from the median part of which, 
the mesothelium of the mandibular arch extends forwards, while 
the lateral part of the mesothelial pocket is continued dorso-laterally 
into the hyoid arch. Cells from the posterior margin of the mandi- 
bular bridge of mesectoderm are seen below this mesothelial tissue. 
The central cavity in the thyroid outgrowth is continuous with the 
cavity of the branchial chamber. At its posterior extremity the 
outgrowth meets the anterior wall of the pericardium, and in this 
region ultimately gives rise to the thyroid gland. The anterior part 
of the outgrowth becomes irregularly broken, and part of the cells 
are finally lost in the mylohyoid musele'. 
1 Paut Busor (91, page 82) claims that in Petromyzon part. of the 
muscles of the lingual piston and pharynx come from cells derived at meta- 
morphosis from the disintegrated thyroid and velum. 
