342 MISS E. A. FRASER ON THE DEVELOPMENT 



In Chelydra, however, the m. retractor bulbi separates ofl'fi'om 

 the m. rectus externus at an early stage before the second somite 

 has completely degenerated and when the m. rectus inferior and 

 internus first begin to difFei-entiate from each other, whereas in 

 Ti-ichosurus it only becomes an independent muscle at 17*5 mm. 

 after the other muscles of the eye have assumed their final 

 positions and at a time when the m. rectus inferior and internus 

 are almost completely separated. The m. retractor bulbi in 

 both animals is the last of the eye-muscles to reach its adult 

 position. 



The cranial nerves III, IV and VI arise from the brain and 

 grow towards their respective muscles, their connection with the 

 mesodermal somites being a secondary one as shown by NeaJ ('14). 

 No independent origins in the muscles themselves are present as 

 described by Filatoif ('07) in Emys lutaria. The oculomotor i-vms 

 at its distal end into the proliferation on the postero-lateral wall 

 of the head-cavity and breaks up into many fibres at the region 

 where the m. obliquus inferior and m.. rectus inferior first grow 

 out, these two muscles being innervated at practically the same 

 time ; later, at the earliest indication of the m. rectus internus, 

 fibres are seen to run into the root of the latter. The abducens 

 develops in the typical manner and runs into the posterior end 

 of the abducens muscle-mass ; when the m. retractor bulbi grows 

 forwards and separates off" from the m. rectus externus, the nerve 

 branches into the two muscles at the point where the foi-mer 

 crosses the medial side of the latter. The trochlear, as is usually 

 the case, arises later than the other two nerves. In Trichositrus 

 it first appears at 8'5 mm, and does not reach the m. obliquus 

 superior until 11 mm,, when it penetrates into the posterior side 

 of the medial end of this muscle. 



Bihliograjyhy. 



'97. Allis, E. p., Jr. — " The Cranial Muscles and Cranial and 

 First Spinal Nerves in Amia ccdva" Journ. of Morph. 

 vol. xii., 1897. 



'02. . " The Lateral Sensory Canals, the Eye Muscles, and 



the Peripheral Distribution of certain of the Cranial 

 Nerves in Mustelus Icevis." Quart. Journ. Micr, Sci. 

 vol. xlv., 1902, 



'78. Balfour, F. M. — "A Monograph on the Development of 

 Elasmobranch Fishes." London, 1878. 



'04. BoEKE, J. — ■ " Beitrage zur Entwicklungsgeschichte der 

 Teleostier. II. Die Segmentierung des Kopfmesoderms, 

 die Genese der Kopfhohlen, das Mesectoderm der Gang- 

 lienleisten und die Entwicklung der Hypophyse bei den 

 Muraenoiden." Petrus Camper, 2 Deel. 19U4. 



