200 The Development of the Eye Muscles in Aeanthias 



Conclusions. 



Broadly considered, it will be seen that the necessary mechanical 

 relations between eyeball and muscle is secured: (1) by a forward 

 growth of processes from the 3nd and 3rd somites, and the development 

 of muscle fibres in them; (2) by a S23reading out of the 1st somite 

 around the eyeball and the development of muscles in its distal portions. 



I wish to call attention to the fact, which so far as I know has not 

 been noticed before, that the original direction of all the eye muscles 

 together with " Muscle E " is longitudinal. This seems to me to repre- 

 sent an originally flexible condition of the head and to be an additional 

 support for the homology of head and trunk somites. 



Finally, it seems to me improbable that the present musculature of 

 the eye in Aeanthias is the primitive one for several reasons: (1) The 

 adult condition is reached only after the constituent muscles have 

 undergone rather extensive alterations in form and transfer of position, 

 (2) The muscles do not all arise equally early, nor do they reach their 

 definitive condition at the same time. (3) Before some of the perma- 

 nent eye muscles are formed, one muscle ("' Muscle P] "), which later 

 disappears, reaches an advanced stage of development. This muscle, 

 from its form and position, must either have once been functionally 

 connected with the eye or with some structure now lost, and of which 

 not even an embryonic rudiment is known. The same reasoning applies 

 to the anterior somite though with diminished force, since it does not 

 reach the same advanced stage of development. 



If then the present musculature of the eye is not the primitive one, 

 it becomes an interesting question to inquire if the embryonic develop- 

 ment will indicate any stages in the phylogenetic development. Two 

 such stages, it seems to me, are indicated. 1st, a stage where if any 

 eye musculature existed it was furnished by the anterior somite. This 

 is indicated first by the fact that this somite is the only one which from 

 its topographical relations could move the eye; and second, the longi- 

 tudinal direction and serial arrangement of the remaining muscle 

 anlagen indicate a jointed condition of the head and consequently a 

 functional activity on the part of these muscles which would preclude 

 any connection with the eye. 



2nd, a stage at which four muscles moved the eye. These were the 

 superior oblique, the external rectus, the inferior oblique and " Muscle 

 E." These four muscles were arranged radially. " Muscle E " and the 

 inferior oblique opposed one another, the former pulling the back of 

 the eye dorsally, the latter, ventrally. The superior oblique and the 



