HISTORY OF THE EYE MUSCLES 437 



Of Miss Piatt's 'muscle E' Lamb ('02, p. 195) says: 



This latter muscle has now (in a 19 mm. embryo of Squalus) reached 

 its maximum development. The anterior end curves not only outward 

 but upward as well, so that the direction of the muscle is approximately 

 dorso-ventral. From now on this muscle undergoes degeneration 

 at as a 26 mm. stage scarcely a trace of it remains. 



Proof of its degeneration, however, Lamb does not give. That 

 Lamb is confused regarding the fate of the muscle is shown by 

 the fact that in his figure 9, p. 185, 'muscle E,' which is cor- 

 rectly so labelled m the drawing, is incorrectly described in the 

 paper as "prohferated from the hyoid somite," while it is named 

 "the external rectus muscle!" Such contradictions are unfor- 

 tunate in a paper which is otherwise a valuable contribution to 

 the literature. 



Johnson ('13, p. 161) apparently assumes the degeneration of 

 the so-called muscle E (which occurs in Reptile embryos in the 

 same relations as in Elasmobranchs) without taking the trouble 

 to inquire into the evidence. He agrees with Miss Piatt and 

 Lamb that it degenerates and, like them, fails to prove the 

 assertion. Miss Fraser ('15), whose paper is the latest deal- 

 ing with the problem of the ontogenesis of the eye-muscles, is 

 more cautious in her statements. She is willing to admit the 

 possibility (p. 341) that in marsupials the second myotome 

 "may contribute towards the formation of the m. externus 

 rectus as in some fishes (Dohrn '04, Neal '09, '14)," but she adds 

 "we have no direct evidence of this in Trichosurus." 



Dohrn ('06) was the first to assert the persistence of the mandib- 

 ular component of the external rectus muscle. He states in a 

 foot-note (p. 243): 



bei Scyllium kann man sich mit. der grossten Sicherheit davon iiber- 

 zeugen, dass von einem Zugrundegenen des mandibularen Antheils des 

 Rectus externus keine Rede ist, da man Schritt flir Schritt die starkere 

 Verdickung der Masse und die Bildung der einzelnen Fasern innerhalb 

 diesser Masse constatiren kann, wahrend gleichzeitig die aus der III 

 Kopfhohle stammenden Muskelfasern immer naher an die der Mandib- 

 ularhohle aufriicken und zugleich von hinten her sparsamer werden, 

 bis schhessHch die ganze Muskelmasse keinen Unterschied mehr dar- 

 bietet. 



