MORPHOLOGY OF EYE MUSCLE NERVES 5 



cranial and spinal nerves which had been rendered doubtful by 

 the incomplete researches of earlier students of nerve develop- 

 ment. The recent elaborate studies of the histogenesis of the 

 trochlear and oculomotor nerves by Dohrn ('07) and Gast ('09) 

 have tended greatly to strengthen the conviction of morpholo- 

 gists that these nerves are comparable with spinal nerves. Belo- 

 golowy's ('10) careful monograph on the cranial nerves of the 

 chick leads to the same conclusion. It is the purpose of the 

 present study to make a contribution to the same end. 



The writer takes pleasure in expressing his appreciation of 

 the aid given him during the prosecution of this research 

 by Professor John Sterling Kingsley, Director of the Harpswell 

 Laboratory, during the summers of 1906-1913 arid for the un- 

 limited supply of embryos of Squalus acanthias which residence 

 at this laboratory has rendered available. He also acknowledges 

 gratefully a grant of $50 by the trustees of the Elizabeth Thomp- 

 son Fund, used in the attempt to get embryos of stages between 

 50 and 100 mm., not obtainable off the Maine coast during the 

 summer months. 



1. The importance of the eye-muscle nerves as criteria of the 

 metamerism of the head 



In a recent paper dealing with the head problem Ziegler ('08, 

 p. 674) writes that, since he holds that the eye muscles arp 

 relatively young muscles which have not arisen directly from 

 segmental muscles, he is of the opinion that one cannot make 

 use of their innervation for phylogenetic conclusions. Ziegler 

 does not give his reasons for regarding the eye muscles as rela- 

 tively young muscles. Moreover, he does not prove that they 

 have not arisen directly from segmental muscles, and in drawing 

 conclusions concerning the phylogenesis of the head he makes use 

 of the relations of the oculomotor as an essential connecting link. 



Cole ('98, p. 237) also refers to the eye-muscle nerves as the 

 ''the least primitive of the cranial nerves" without giving any 

 proof of the assumption. 



