MORPHOLOGY OF CEANIAL MUSCLES IX SOME VERTEBRATES. 181 



and constrictor spiraculi — all innervated by the Ytli. Kide- 

 wood supposed that the first-named was differentiated from the 

 same tract as the levator maxillte superioris and that the 

 second belonged to the same category, whilst the constrictor 

 spiraculi appeared to belong to a purely dermal system of 

 muscles. 



According to Harman, the muscuhiture of the eyelids of 

 Mustelus arises "from two original sources — one a superficial 

 dermal layer, the other a portion of a deeper dermal muscle 

 layer," the former originating from " a mass which appears 



Text-fig. 9. 



tvtax-i-ujJL 



Scyllium. emhiyo 20 mm., longitudinal horizontal section thronijli 

 first branchial segment on left side. 



first in the region of the branchial bar Avhich is the second 

 after the spiracle," the latter from a mass which separates 

 into "a maxillary mass and a spiracular mass." Harman 

 also stated that "the progressive growth of the spiracle 

 muscles in the service of the face can be traced to their full 

 development in the facial muscles of man." 



In Scyllium the Anlage of the nictating muscles can be seen 

 in 30 mm. embryos (Text-figs. 16, 17) as a mass of cells being 

 proliferated from the outer surface of the upper end of the 

 levator maxillee superioris. In 40 mm. embryos the mass 

 has become partially divided into a deeper and a more 

 superficial layer — the former is the Anlage of the levator 



