538 F. H. EDGEWORTH. 



Lacertilia vera with the upgrowth of the external ptery- 

 goid muscle, to the side of the skull. Investigation of the 

 process of development of the head-muscles did not throw 

 any light on the causation of the fixation of the pterygo- 

 quadrate in Chelonia, on the fixation of the quadrate in 

 Rhiptoglossa, or its movability in Ophidia. 



The above differences, however, in the condition of the 

 upper part of the mandibular myotome and of the pterygoid, 

 suggest that fixation of the pterygo-quadrate was brought 

 about by different processes in, and so was independently 

 acquired by, ancestors of (1) Chelonia, (2) Crocodilia, 

 (3) Rhyncocephalia and Lacertilia vera, (4) Rhiptoglossa 

 and Ophidia. 



Myotome of the Hyoid Segment. — The abducens 

 musculature of Gallus originates as a small clump of cells 

 which grows forward and then separates from the upper end 

 of the hyoid myotome. It passes forwards internal to the 

 Gasserian ganglion (text-fig. 4, p. 514), and divides into an 

 anterior and a posterior mass (text-figs. 10 and 14, pp. 518, 

 521) ; the anterior is the Anlage of the pyramidalis and quad- 

 ratus nictitantis muscles, the posterior that of the external 

 rectus. In Reptiles the abducens Anlage similarly divides into 

 an anterior and a posterior mass ; the former (except in Tropi- 

 donotus, where it atrophies) develops into the retractor oculi,^ 

 the latter into the external rectus (text-figs. 22, 29, 30, pp. 

 527, 532, 534). 



The myotome of the hyoid segment, after giving off the 

 abducens Anlage, develops into the depressor mandibulas. 

 The primary insertion of this muscle is into the ceratohyal 

 (text-figs. 12, 18, 24, 30, and 32, pp. 519, 525, 528, 534, 

 536), but this is soon lost, and one into the hind end of the 

 lower jaw acquired. In Galhis (text-fig. 14, p. 521), Alligator, 

 and some Lizards (Parker), a stapedius muscle is also formed 



• The retractor oculiis greatly developed in Sphenodon, its origin extend- 

 ing backwards (text-fig. 28, p. 531). Its insertion varies, and accordingly 

 Huxley calls it "pyramidalis" in Chelonia and Crocodilia, " bursalis" in 

 Lizards. 



