34 GENERAL EMBRYOLOGY. 
is formed, which projects, forwards and downwards, towards the pericardium. This 
is the first indication of the stomach. 
Visceral Clefts and Visceral Arches.—In the lateral wall of the anterior part 
of the fore-gut, on each side, four incomplete and more or less transverse clefts, the 
visceral clefts, appear. They are due to outward linear pouchings of the entoderm, 
and corresponding, but less marked, inward depressions of the ectoderm. The 
anterior cleft is the best marked, and the rest diminish in size from before back- 
wards. At the bottoms of the clefts the ectoderm and the entoderm are in contact, 
but the thin membranes thus formed, which intervene between the cavity of the 
fore-gut and the exterior, are only exceptionally and abnormally perforated in the 
human subject, though in lower vertebrates they invariably disappear, and the 
pharyngeal or anterior part of the fore-gut is thrown into continuity, laterally, 
with the exterior by a number of narrow slits, the gill slits, which are used for 
respiratory purposes. In man and other mammals, however, the floors of the 
second, third, and fourth clefts are utilised in the formation of the sides of the 
neck; that of the first cleft is transformed into the tympanic membrane, which 
separates the external auditory meatus from the cavity of the tympanum. 
In the further consideration of the fate of the visceral clefts, 1t must be borne 
in mind that each consists of an inner or entodermal portion and an outer or ecto- 
dermal portion. The inner part of the first cleft is converted into the tympanum 
and the Eustachian tube, and the outer part becomes the external auditory meatus. 
No traces of the outer part of the second cleft are left, but a portion of the imner 
part can be recognised as a slight depression above the tonsil in the lateral wall of 
the pharynx. Both the outer and inner portions of the third and fourth clefts 
disappear, but from their inner parts diverticula are given off which form the 
rudiments of the thymus and the lateral lobes of the thyroid body. The 
diverticula from which the thymus is developed are derived from the third clefts, 
whilst each lateral lobe of the thyroid body is formed by a diverticulum from the 
fourth cleft. 
The margins of the visceral clefts are thickened by the growth of the mesoderm 
between the entodermal and ectodermal layers, and they are moulded into a series 
of five rounded bars, the visceral arches, of which the fifth is not recognisable 
externally, though it is easily seen internally. The dorsal extremities of the arches 
terminate at the sides of the head below the level of the neural tube, and in the 
early stages the ventral ends rest upon the pericardial region. When the neck is 
formed, it grows forwards from the pericardial region and carries with it the lower 
ends of the visceral arches, which henceforth terminate in its ventral wall. As the 
visceral arches are carried forwards the head is strongly curved towards the ventral 
aspect, and the lower ends of the visceral arches are pushed backwards over each 
other till the fourth is overlapped by the third, and the third by the second. 
The first arch is the mandibular, the second the hyoid, the third the thyro-hyoid ; 
the fourth and fifth have no special designations. Each arch is covered—externally 
by ectoderm, internally by entoderm, and its core is formed of mesoderm, in which 
there is developed a bar of cartilage and a blood vessel called a cephalic aortic 
arch. 
At first each arch is limited to the side wall of the fore-gut; but after a time it 
is prolonged into the ventral wall, encroaching, with the exception of the first, upon 
the sinus arcuatus. 
The first, or mandibular arch, is formed between the first visceral cleft and the 
bucco-pharyngeal membrane. As it develops it forms the lateral and lower 
boundaries of the stomatodzeal space, and it grows downwards till it meets its 
fellow of the opposite side in the ventral middle line, immediately in front of the 
tuberculum impar. The greater part of this arch is converted into the lower jaw 
and the soft tissues which invest it. From its upper part a process grows forwards, 
the maxillary process, from which the upper lateral part of the face, between the 
orbit and the mouth, is developed, and in which the superior maxillary, the malar, 
and the palate bones, and possibly the internal pterygoid plate also, are developed 
and ossified. 
From the posterior border of the outer aspect of the mandibular arch the tragus 
