128 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
epaxial and hypaxial muscles are clearly recognizable, but farther 
forward the hypaxials are greatly reduced, and in the amniotes the 
reduction is carried so far that the hypaxial muscles, greatly modified, 
can only be recognized in the cervical and pelvic regions. 
In the head the developmental conditions are more complicated 
than in the trunk, our information being most complete with regard to 
the ichthyopsida. Here, in the region which develops into the head, 
ten coelomic pouches are developed (in amniotes the number is appar- 
ently twelve). These are known by number, except that the most anter- 
ior, which was not known when the numbers were applied is called A. 
These ccelomic cavities (also known as head cavities) differ from the 
myotomes farther back in having no undivi- 
ded portion of the coelom below, correspond- 
ing to the hypomeral zone, a difference possi- 
bly due to the existence of visceral clefts in 
this region (fig. 136). 
Four of these cavities lie in front of the ear. 
Of these A disappears completely, its cells 
joining the mesenchyme, while the other three 
Fic. 137.—Diagram of give rise to the ‘eye muscles’ which move 
the eye muscles of the right * : : 
eye, teen fern die tostial. ooo eye Dall. Without going into all of the 
side. er, external rectus; ifr, details, 1, which lies in front of the mouth, 
ablinue: Te iniemtl recta, gives rise to the superior oblique muscle; 2, 
so, superior oblique; s7,supe- which lies in the region of the jaws, forms four 
rior rectus; JZZ, coulomotor; a 7 ; 
IV, trochlearis; VI,abducens muscles, the inferior oblique and three of 
RSENS? the rectus muscles (in some forms also a 
retractor bulbi), while 2, in the hyoid region, develops the external 
rectus. This method of origin explains the distribution of the eye- 
muscle nerves to be described later, each nerve supplying only the 
derivatives of a single myotome. Several of the other head myo- 
tomes disappear in development, while the posterior form the so-called 
hypoglossal musculature (fig. 138). 
In the above account there is given merely the origin of the con- 
tractile tissue of the muscles. To this other parts of connective tissue 
are added. Mesenchyme cells invade the masses of muscle fibres, 
forming envelopes (perimysium) which bind the fibres into bundles 
(fasciculi) which, in turn, are united by other envelopes, the fascia. 
These connective-tissue envelopes are extended beyond the contractile 
tissue and form the cords or tendons by which the muscle is attached to 
