448 H. V. NEAL 



(Reigard '02) they open to form the suckers of the Ganoid larva. 

 Similarly in Balanoglossus, it will here membered, the anterior (pro- 

 boscis) cavities open to the exterior by the proboscis-pore, which 

 is sometimes right and sometimes left in its position. -Since these 

 cavities are the most anterior in the chordate body, since they lie 

 immediately anterior to the first permanent myotomes and since 

 they open — or at least one of them opens — to the exterior in Am- 

 phioxus and a Craniote (Amia), and since they are peculiar in 

 this respect, their exact homology seems not unreasonable and 

 strengthens the assumption of the homology of the first per- 

 manent myotomes in Craniotes and Acraniotes. 



Assuming, therefore, on the basis of this evidence, the exact 

 homology of the latter, the history of the eye-muscles may be 

 seen to be the history of the transformation of the first three myo- 

 tomes of an Amphioxus-like ancestor into the definitive six eye- 

 muscles of man. This history may be very briefly summarized: 

 Primarily, as in Amphioxus, the three anterior myotomes were 

 members of an unbroken series of segmented muscles extending 

 throughout the entire length of the body. When lateral line 

 organs and enlarged cranial ganglia associated with them made 

 their appearance, the anterior myotomes became split length- 

 wise into dorsal and ventral moieties. Further separation and 

 displacement followed the enlargement of the optic and otic 

 vesicles. In this way eventually two sets of muscles, one dorsal 

 and one ventral, were brought in close proximity to the enlarg- 

 ing optic vesicles with which they finally became functionally 

 associated (figs. 17 and 18). 



How these two sets of muscles became gradually transformed 

 into the eye-muscles is revealed by their ontogenesis in Elasmo- 

 branchs. Each of the divisions (dorsal and ventral) of the first 

 myotome divides again, thus forming the four muscles innervated 

 by the oculomotor nerve. The second myotome undergoes no 

 further subdivision. Its dorsal moiety becomes the superior 

 oblique muscle and is innervated by the trochlearis nerve. Its 

 ventral portion, however, unites with the third myotome to form 

 the external rectus muscle and becomes innervated by the ab- 

 ducens nerve. The dorsal moiety of the third myotome does 



