402 Julia B. Platt 
the left end is higher than the right, it may possibly be fitted in 
imagination to the embryo of fig. 1, so that the relation of the mes- 
ectoderm to the surface of the embryo becomes more evident. 
The fore-brain passes over the curved anterior margin of the 
mesectoderm (fig. 2 fb). The depressions marked »r« are where 
the thickened nasal epithelium indents the underlying tissue. Above 
the nasal indentations, the margin of the mesectoderm is hollowed 
out as the tissue passes around the eye. Along the margin marked 
»oph«, the main branch of the ophthalmicus profundus trigemini 
lies. This ganglionic nerve is formed like the proper cranial ganglia 
from cells cut off from the dorsal part of the mesectoderm. From 
the anterior extremity of the figure to the hyomandibular cleft, the 
tissue may be called »trigeminal<, since derived from that anterior 
part of the neural crest, which, with its secondary ectodermie addi- 
tions in dorso-lateral and epibranchial lines, gives rise to the gang- 
lionie cells of the trigeminal group in its dorsal part, and to the 
mesectoderm of the anterior part of the head and of the mandibular 
arch in its ventral part. 
Throughout the space marked »m« (fig. 2) the ectoderm and 
endoderm have fused. In the anterior, wider, part of the space, 
the mouth ultimately appears. The posterior, narrower, part of the 
space gradually closer by the continued growth in an anterior direction 
of the tissues that unite the right and left halves of the mandibu- 
lar arch ventrally. The oral fusion of ectoderm and endoderm is 
primarily throughout an axially elongated space from which there 
are no transverse, or lateral, extensions, such as are found at the 
anterior part of the fusion in fig. 2. As the oral fusion extends 
laterally it becomes reduced posteriorly so that the mouth finally 
breaks through in the transverse plane. In the embryo of fig. 2, the 
fusion of the mesectoderm of each half of the mandibular arch with 
that of the opposite side has just begun. 
The anterior part of the trigeminal mesectoderm, which is partly 
separated from the posterior or mandibular part by the oral fusion, 
has united widely (fig. 2) with the corresponding mesectoderm of the 
opposite side of the head, forming a shell of tissue, that is separated 
from the floor of the anterior part of the brain merely by a thin 
layer of reticular mesenchyma, and which lines the ectoderm of the 
anterior (dorsal) wall of the mouth. Figs. 1c, g, 4, and «, show that 
the trigeminal mesectoderm gradually approaches the axial plane on 
the anterior wall of the buccal cavity. In sections immediately 
