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3 The Mesodermic Somites. 
The first mesodermic somites to be formed lie in the region 
marked 10, fig. 3. Whether the very somite thus designated or an 
adjacent one be first formed, I am unable to say. Anterior to this 
region, at an earlier stage than that represented by fig. 3, the meso- 
derm contains no cavity, but consists of two lateral plates fusing in 
the median line with the endoderm in which the anterior end of the 
chorda is also lost. Later three pairs of cavities appear in the anterior 
portion of the mesodermic plates (fig. 3 2, 2’ and 3). The posterior 
of these cavities is known as the third head cavity (vAN WiJHE'), 
and from its walls is formed the external rectus eye-muscle, supplied 
by the abducens. The two anterior cavities fuse into one, and are 
known as the mandibular cavity, from the walls of the anterior portion 
of which the superior oblique eye-muscle is formed, together with 
another large muscle, lost in early embryonic life. This portion of 
the mandibular cavity is supplied by the trochlearis. From the walls 
of the posterior portion of the mandibular cavities are formed the 
proper mandibular muscles, which are supplied by the trigeminus. 
The walls of these three cavities, 2, 2’ and 3, therefore, give rise to 
distinct voluntary muscles, supplied by separate nerves. The walls 
are further formed from the mesodermic plates which give rise to the 
somites of the posterior portion of the head and to the body somites. 
I therefore consider each of the three cavities the homologue of a body 
somite, since they resemble the body somites in size, position, and 
subsequent development. 
In my former paper (loc. cit.) I have discussed the development 
of the muscles formed from the walls of the anterior cavities (2, 2 and 5) 
and will here only allude to the fact that the muscle cells appear 
first in the inner walls of the cavities, resembling, in their place of 
origin, the origin of the muscle cells of the trunk. 
At the stage represented in fig. 3, no cavities have appeared in 
the mesoderm between the somites marked 3 and 9 respectively, but 
depressions in the dorsal wall of the mesoderm indicate the probable 
existence of three intervening segments. The mesodermic plate has 
now lost its connection with the endoderm, save in the anterior portion 
of the mandibular cavity. Cross sections through this region show an 
exceedingly interesting relation to exist between the mandibular cavities 
and the alimentary canal, inasmuch as the two appear to be in com- 
1) van Wisner, „Über die Mesodermsegmente und die Entwickelung 
der Nerven des Selachierkopfes“. Amsterdam 1882. 
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