HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 161 



vanced development but thereupon degenerates and disappears. 

 Lamb observed this muscle in Acanthias embryos, and in his 

 illustrations its position and relations to the M. obliquus superior 

 and the muscles of the mandibuMr arch, are strikingly similar 

 to the conditions in Chelydra. It seems to me entirely probable 

 that in the ventral portion of the second head somite of Chelydra 

 we have the vestige of a muscle corresponding to this passing 

 muscle appearing in elasmobranchs, the past functional signi- 

 ficance of which is unknown. 



The oculomotor muscles. The first muscle of this group to 

 appear is the M. obliquus inferior. It begins at about the 9-mm. 

 stage, as a somewhat loose outgrowth of cells from a rather small 

 area on the outermost ventral portion of the wall of the first 

 head somite, after the somite has reached its maximum expan- 

 sion. The outgrowth is not in the nature of a true evagination 

 of the wall, but irregular spaces from the somite cavity extend 

 into its base. It takes a direction anteriorly and slightly ventrad 

 towards the ventral surface of the eye-ball. By the 10-m.m. 

 stage it has become a solid club-shaped mass, the narrow proxi- 

 mal end of which still has a loose cellular connection with the 

 somite wall. From this position it slowly moves forward along 

 the medio-ventral surface of the eye-ball, and in a stage repre- 

 sented by an embryo of 8.5-mm. carapace length, the end which 

 has become free from the somite wall becomes attached to the 

 eye-ball, forming the insertion of the muscle; its opposite, ante- 

 rior end has turned toward its attachment of origin. While it 

 is the first of the oculomotor muscles to appear, it is the last to 

 receive its nerve supply, and is preceded by the M. rectus superior 

 in establishing its insertion. 



The M. rectus superior arises soon after the M. obliquus infe- 

 rior. Its anlage appears as a broad thickening of the posterior 

 wall of the first head somite, near the dorsal side, extending 

 from near its lateral end medially about three-fourths the width 

 of the somite. Its outer posterior side is closely covered by the 

 ciliary ganglion. From the beginning the position of this muscle 

 is such that but a slight change in direction is necessary in reach- 

 ing its definitive position. It bends antero-dorsally as it ap- 



