No. 3-] THE VERTEBRATE HEAD. 579 



due to the same general cause as the neural segments. I am 

 of the opinion that the latter alternative is the correct one. 



When first formed, the auditory saucer is opposite the ninth 

 neuromere; but subsequently, while it is still in the saucer 

 condition, it shifts backwards, and comes to lie at the side of 

 the tenth neural segment, and later still, as a capsule, it lies 

 opposite the eleventh neuromere. 



The history of the auditory vesicle in sharks has been worked 

 out beyond this period by Ayers ('92), and it is very clear from 

 its mode of growth that it is directly related to the canal organs 

 of the lateral line. 



A consideration of the so-called branchial sense-organs and 

 their ganglia is reserved to be published later with the part on 

 the Nerves. 



Note. — I have been indebted to the persons mentioned below for ma- 

 terial that has helped fill up the gaps in my collection of embryos, and I 

 desire here to express my appreciation of their kindness. 



To Miss Julia B. Piatt, for the loan of sections of Acanthias ; to Dr. E. 

 L. Mark, for young stages of Squalus ; to Mr. Frank Smith, for a consid- 

 erable number of embryos ; to Professor J. E. Reighard for the loan and 

 free use of his entire collection of elasmobranch embryos ; to Professor 

 A. D. Morrill, for early stages of Squalus. 



Lake Forest, Illinois, 

 January, 1895. 



