500 LOCV. [Vol. XI, 



specimens of all ages and sizes. Nevertheless there are gaps 

 in the collection and these are often of a kind that it may not 

 be easy to fill in. No two embryos are exactly alike in all the 

 pictures which they yield of this apparatus, and in half a dozen 

 specimens that would be taken to be of the same age from 

 their sizes, from a comparison with Balfour's stages, or from 

 the more certain criteria of number of gill-clefts, protovertebras, 

 etc., etc., it is quite common to find this transient nervous 

 system, like other organs, in widely different stages of devel- 

 opment and presenting great variations in detailed characters. 

 In my researches on Raja I have been compelled to give up 

 completely any attempt to make use of Balfour's nomenclature. 

 With a limited number of embryos at one's disposal, it is 

 easy to fit them into one or the other of the well-known stages ; 

 with an increased number this becomes more difficult or even 

 impossible, so great are the variations met with." I have had 

 a similar experience with my embryos of Squalus acanthias 

 {Acanthias vulgaris). I am conscious there are gaps in the 

 material, and, after making the best use of it that I can, I 

 have, no doubt, missed many things not represented in my 

 collection. 



The studies recorded in this paper deal almost exclusively 

 with the early history of the brain and sense-organs. Some of 

 the questions upon which direct evidence is brought are : 

 What was the primitive condition of the nervous system of 

 the Vertebrates .-" What was the number and nature of the 

 primitive neural segments entering into the brain } What has 

 been, in general, the line of modification along which they 

 have been converted into the brain } And what were the early 

 steps in the differentiation of sense-organs .-' 



I have been greatly indebted to Professor C. O. Whitman 

 for courtesies both at Woods Holl Biological Laboratory and 

 at the University of Chicago, and I have also to thank him for 

 advice and suggestions on the subject-matter of this paper.^ 



1 Soon after the appearance of the Zieglers' paper (Reitrage zur Entwicklungs- 

 geschichte von Torpedo: Archiv fiir Mik. Anat., Bd. 39, Hft. i, Jan. 1S92) Dr. 

 Whitman suggested to me that I should work over the same ground — the gastru- 

 lation and formation of the germinal layers — in one of our North American 



