56 ANNALS NEW YORK ACADEMY OF SCIENCES 



tain early stages of development. They describe the origin of the human 

 jaw muscles that are innervated by the fifth nerve as follows : 



In the 7 mm. embryo the mandibular arch is filled with a closely packed 

 mesenchyme, with only slight traces of condensation about the peripheral end 

 of the mandibular nerve. In the 9 mm. embryo, however, this condensation is 

 clearly to be recognized. This peripheral mass, in which the N. mandibulars 

 ends, lies at about the middle of the arch. In a 11 mm. embryo, this egg- 

 shaped pre-muscle mass has increased in size but still shows no indications of 

 splitting into the various muscles. The pre-muscle is, from the beginning, 

 closely associated with the condensed mesenchyme of the mandible, and with 

 the differentiation of the proximal end of the mandible, the pre-muscle mass is 

 partially split into a Y-shaped mass, the handle representing the M. temporalis, 

 the outer limb corresponding to the M. masseter, and the inner, deeper limb, 

 separated from the outer by the proximal end of the mandible, representing 

 the mass for the Mm. pterygoidei externus and internus. 



Thus it is seen that all these jaw muscles arise from the same group. 



The relations of the jaw muscles to the body muscles as a whole are as 

 follows: In the early stages of the vertebrate embryo the myotome is a 

 hollow body with a splanchnic inner layer and an outer or somatic layer. 

 This myotome grows down until it almost meets its fellow on the opposite 

 side, the two remaining separated from each other by a connective tissue 

 septum, the linea alba of the fishes. The myotome is also divided by a 

 longitudinal line of connective tissue that follows along the lateral line 

 of the fishes and the urodeles and divides the myotome into a dorsal or 

 epaxial epimere and a ventral or hypaxial hypomere. The muscles of the 

 body are accordingly divided into three groups — appendicular, axial and 

 visceral. The appendicular muscles take their origin from the axial, and 

 both in turn are derived from the dorsal or epaxial (epimere) part of the 

 myotome. The visceral muscles, including those of the head region, are 

 derived from the ventral or hypaxial portion of the myotome (hypo- 

 meres). (Wilder, 1909, p. 190.) The visceral or hypaxial muscles of 

 the head region are further divided into a dorsal and a ventral series 

 according to their position as depressors or levators of the gill arches. 



NERVE SUPPLY 



The primitive innervation of the myotomes in Amphiozus is shown 

 by Goodrich (1909, p. 2). The myotomes are supplied by the ventral 

 roots of the serial nerves, the dorsal roots passing between the myotomes 

 to supply the skin and the sensory nerves. The dorsal and ventral roots 

 of the serial nerves remain separate. Eegarding the innervation of typical 

 vertebrates Goodrich (1909, p. 3) says: 



