The tongue is a product of four branchial arches,
whose ventral ends merge in its midplane. Papillae elevate from the surface,
and taste buds arise as specializations within the covering epithelium of
some of them. Pharyngeal pouches are early lateral expansions of the local
endoderm, alternating with the branchial arches. The first pair elongate as
the auditory tubes and tympanic cavities. The second pair mark the site of
the tonsils. The third pair give rise to the halves of the thymus, and the
third and fourth pairs produce the two sets of parathyroid glands. The
thyroid gland buds off the pharyngeal floor in the midplane and at the level
of the second branchial arches.
As the embryo folds off, the endoderm is rolled in
as the foregut and hindgut. Continued growth progressively closes both the
midbody and the midgut (Figure 1O). The esophagus remains as a simple,
straight tube. The stomach grows faster on its dorsal side, thereby forming
the bulging greater curvature; the stomach also rotates 90° so that its
original dorsal and ventral borders come to lie left and right. The
intestine elongates faster than the trunk, so that its loops find temporary
room by pushing into the umbilical cord. Later, the loops return, completing
a rotation that gives the characteristic final placement of the small and
large intestines.
When the gut folds into a tube it is suspended by a sheetlike dorsal
mesentery, or membranous fold. In the region of the stomach it forms an
expansive pouch, the omental bursa. Secondary fusions of the bursa and of
some of the rest of the mesentery with the body wall produce lines of
attachment from stomach to rectum inclusive, different from the original
midplane course. Such fusions also anchor firmly some parts of the tract. A
ventral mesentery, beneath the gut, exists only in the region of the stomach
and liver.
The liver arises as a ventral outgrowth of the
foregut that invades the early transverse septum. Although rapid growth
causes it to bulge prominently away from this septum, it remains attached to
the septum and hence to the definitive diaphragm. The differentiating
glandular tissue takes the form of plates bathed by blood channels. The stem
of the original liver bud becomes the common bile duct, whereas a secondary
outgrowth produces the cystic duct and the gallbladder.
The pancreas takes its origin from a larger dorsal bud and a smaller ventral
bud, both off the foregut. The two merge and their ducts communicate, but in
man it is the lesser, ventral duct that becomes the stem outlet. Secretory
acini are berrylike endings of the branching ducts. Pancreatic islets arise
as special sprouts from the ducts; these differentiate into endocrine tissue
that secretes insulin.
The first part of the respiratory system is
ectodermal in origin. The olfactory sacs become continuous secondarily with
a passage captured from the primitive mouth cavity. This addition is
produced by a horizontal partition, the palate. It arises from a pair of
shelflike folds that grow out from the halves of the primitive upper jaw and
then unite. The final nasal passage extends from the nostrils to the back of
the pharynx.
A hollow lung bud grows off the floor of the
endodermal pharynx, just caudal (tailward) to the pharyngeal pouches and in
the midline. It has the form of a tube with an expanded end. The entrance to
this tube is the glottis, and the region about it becomes the larynx. The
tube proper represents the trachea. Its terminal expansion divides into two
branches, and these tubes elongate as the primary bronchi. Continued growth
and budding produce two side branches from the right bronchus and one from
the left. These branches and the blind ends of the two parent bronchi
indicate the future plan of the lungs, with three right lobes and two left
lobes. Continued branchings, through the sixth month, produce bronchioles of
different orders. In the final months the smaller ducts and early
respiratory alveoli (air sacs) appear, the lungs losing their previous
glandular appearance and also becoming highly vascular. Until breathing
distends the lungs, these organs remain relatively small.
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