No. i.] MORPHOLOGY OF VERTEBRATE HEAD. 87 



ment of which appeared in the walls of the mandibular cavity 

 prior to the origin of any of the eye-muscles, completely disap- 

 pears, although in the embryo of 22 mm. it is still relatively 

 large as compared with the eye-muscles. 



Fig. 5, PI. IV, shows the eye-muscles of an embryo of 27 mm. 

 Between this stage and that last reconstructed (Fig. 4) the out- 

 line of the anterior head cavity has become indistinct, and its 

 cells, undergoing retrogressive development, have lost their 

 muscular character and are finally indistinguishable from the 

 cells of the general mesoderm which surrounds the eye. We 

 have reason therefore to believe that two large muscles are lost 

 in this region of the head ; namely, that formed from the anterior 

 head cavity and the rudimentary muscle of the mandibular cav- 

 ity (mus. e.). What the function of these muscles may have 

 been I am unable to guess, unless they possibly belong to the 

 lost gill arches said once to have existed anterior to the man- 

 dibular arch. In Fig. 5 the premandibular cavity is still found. 

 The four muscles to which its walls give rise are beginning to 

 radiate towards the points of their ultimate attachment. The 

 cells of the rudimentary muscle (mus. c), so closely related to 

 the external rectus, are now indistinguishable from the sur- 

 rounding mesoderm, and the superior oblique eye-muscle repre- 

 sents all that is left of the walls of the dorsal part of the 

 mandibular cavity. 



In an embryo of 35 mm. (Fig. 6) the premandibular cavity 

 has disappeared. The principal changes in the position of the 

 eye-muscles relate to the external rectus and to the superior 

 and inferior oblique muscles. What was originally the posterior 

 limit of the external rectus now lies anterior to the remainder 

 of the muscle. The superior and inferior oblique muscles are 

 approaching one another. In still older embryos the approxi- 

 mation is continued until the points of their proximal attach- 

 ment meet. 



In the embryo of 55 mm. (Fig. 7) the eve-muscles conform so 

 closely to the adult condition that it is unnecessary to trace 

 their development further. 



In concluding, I would say in regard to the anterior head 

 cavities that their primary position and the time of their origin 

 both point to their serial position as between the mandibular 

 and premandibular cavities. Van Wijhe suggests that the 



