No. 3-] THE VERTEBRATE HEAD. 533 



material. I have never seen a shark embryo, of the proper age, 

 of any method of preparation, in which I could not detect 

 them. A dead-black background is, of course, the best surface 

 for observing anything of this kind by reflected light. A white 

 surface is occasionally recommended, but it is desirable to cut 

 off all reflected rays except those coming from the specimen, 

 and one may see delicate structures on a black surface that 

 cannot be seen at all on a white background. 



The care with which the specimens are prepared makes a 

 great difference in the clearness with which these structures 

 may be seen. The conditions under which most material is 

 hardened are unfavorable. There is usually an albuminous 

 fluid in contact with the embryo, and this, together with 

 minute fragments of yolk, coagulates when the fixing reagent 

 is used, and forms a coating over the embryo. The embryos 

 should be washed by a very gentle jet of the reagent immedi- 

 ately after their immersion, and the clouded reagent should be 

 removed and replaced by clear fluid. This makes the prepara- 

 tions beautifully clear. 



2. Are they Artifacts ? 



There is one question that must be answered for all new 

 structures, viz., are they artifacts — produced by the reagents 

 used .-* Too great precaution cannot be taken in considering 

 this question. 



I have used the following reagents in preparing my material: 

 picro-sulphuric acid, picro-nitric acid, Flemming's stronger solu- 

 tion, Davidoff' s corrosive-acetic, chromic acid with a trace of 

 osmic, corrosive sublimate removed with iodine. I have 

 observed Acanthias material prepared by all these various 

 methods, and in every case have found the segments as I have 

 described them. I have always taken the precaution to count 

 the number in the head region, and uniformly have found the 

 same number of segments there, regardless of the hardening 

 reagent used. I have had a complete parallel series of speci- 

 mens hardened with Davidoff's corrosive-acetic and Kleinen- 

 berg's picro-sulphuric, and have compared the two, step by step, 



