408 Julia B. Platt 
internus, which is thus seen to be a muscle belonging to the third 
branchial arch. The mesectoderm of the glossopharyngeal arch 
now begins to show a closer grouping of the cells in the anterior 
part of the arch. 
In fig. 5 5, the median bridge of mesectoderm extends from the 
branchial floor to the wall of the pericardium, and the cells begin 
to be more closely grouped in the axial line, between the two vas- 
cular spaces. The third branchial cleft lies above the plane of section, 
but its position is indicated by elevations in the ectoderm posterior 
to the hyobranebial cleft. As the embryo grows the branchial clefts 
bend backwards from the transverse plane in which they open 
outwards in the younger embryo, and this deflection is most marked 
in the hyobranchial cleft. From comparison with fig. 1 g, it seems 
probable that some of the cells bounding the anterior wall of the 
branchial cavity (the mouth) in fig. 5 4, are of ectodermie origin. 
The difference between the mesectodermic and mesodermic tissues 
in the older embryo, however, is not sufficiently marked to enable 
me to determine with certainty which of the cells in this region 
belong to the mesectoderm. 
Fig. 5 c gives the transition from fig. 5 5 to fig. 5 d, showing 
that below the plane of fig. 5 6, when the hyobranchial clefts have 
passed out of section, the dense mesectoderm of the hyoid arch 
becomes continuous from side to side. Fig. 5 ¢ further shows that 
below the plane in which the mesothelium of the glossopharyngeal 
arch unites with the pericardium, the mesectoderm cells of this arch 
and those in the axial line between the median vascular spaces, 
are widely separate where in fig. 5 6 they were closely grouped. 
The mandibular muscle in passing beyond the transverse plane of 
the oral pocket loses its connection with the wall of the branchial 
chamber and becomes entirely surrounded by mesectoderm, which 
is most dense in the angle between the oral pocket and the floor of 
the hyomandibular clefts. ) 
Fig. 5 d, like fig. 1 4, shows a section through the lower part 
of the eye, and through the point where the mesothelial or muscular 
tissue of the hyoid arch unites with the wall of the pericardial 
cavity. Median to the point of union, in the older embryo, two 
muscle bands extend forwards to the floor of the branchial chamber. 
These muscles form the anterior extremity of the ventral longitudinal 
muscle of the trunk, and their origin is described in the third part 
of this paper. The section given in fig. 5 d, shows the present 
