These _Globular processes_ are separated in the median line by a groove
which is subsequently obliterated by their amalgamation to form the
central portion of the upper lip (“Philtrum” of German authors), and from
their deep aspect the inner segments of the intermaxilla (endognathia).
Above this groove is a central flattened median portion of the
fronto-nasal process, from which subsequently the prominence of the nose
is developed (Figs. 31A, 32, and 33), a result of the continued growth
from its deeper aspect of the ethmo-vomerine plate.
The _external nasal process_ forms the ala nasi and the soft parts in
its immediate neighbourhood, extending downwards a little distance, but
probably not so far as the red margin of the lip. From its deeper aspect
the outer portion of the intermaxilla (mesognathion) is developed, and in
it appears the germ of the lateral incisor. It also serves to separate
the nasal pit from the second epiblastic intrusion which assists in the
formation of the eye. The lower boundary of the naso-orbital fissure
which passes from the primary ocular involution to the stomodæum is
formed by the _maxillary process_, which is usually described as an
upward extension of the mandibular process, but which probably arises
separately as a pre-oral branchial outgrowth. From this is developed
the whole of the superior maxilla, except that portion which is
intermaxillary, and also the greater part of the cheek.
[Illustration: FIGS. 32 AND 33.—Later stages of development of fœtal
head.]
About the sixth week the stomodæum shows signs of division into upper and
lower segments by the outgrowth from the deep aspect of each maxillary
process of horizontal _palatal plates_, which by their junction in the
median line form the rudiments of the hard and soft palate, separating
thus the nasal and buccal cavities. The anterior portion of these unite
with the lateral aspects of the deeper parts of the fronto-nasal process,
leaving a space of greater or less dimensions in the median line, known
as the anterior palatine canal, which serves subsequently for the passage
of nerves and vessels, and for the lodgment of the “organ of Jacobson” in
animals in which it occurs.
The upper or nasal cavity is again subdivided into lateral halves by the
growth downwards from the under surface of the fronto-nasal process of a
central vertical septum, to become in time the cartilaginous septum nasi
and the bony ethmo-vomerine plate, uniting at its lower border with the
primary fleshy palatine processes (Fig. 34).
The tongue grows as a fleshy protuberance from the floor of the
stomodæum; antero-lateral segments on either side from the conjoined
second and third branchial arches unite with a central posterior growth
from the tuberculum impar in an inverted 𝖸-shaped manner. At the point
of junction of the segments is a depression from which the thyroid gland
develops, indicated in later life by the foramen cæcum.
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