Developmental liistory of primary segmeuts of the vertebrate head. ■ 403 



water of rivers where the eggs at hatcheries are reared. Growth 

 therefore was retarded so that embryos 25 days old were approxi- 

 mately developed no farther than those 20 days old at the hatcheries. 

 An allowance of 4 to 7 days must be made if the age of the embryos 

 described is compared with the age of embryos that develop in hatch- 

 eries at the normal or higher temperature. 



The gelatinous envelope of the Salmo egg was removed before 

 the embryo was killed with the fixing reagent. The living embryos 

 sketched were numbered and the observation repeated upon the pre- 

 served specimen. In dissecting preserved material it was soon dis- 

 covered that the most desirable specimens to begin work upon were 

 embryos with 35 to 40 somites where the neural segments of the 

 medulla are very prominent. These segments were then traced through 

 a close series of younger stages as well as in older forms. Further- 

 more, as the segments appear as a rule more distinctly on the inner 

 aspect of the brain wall, it became desirable to expose this surface. 

 This can be done by dissecting away one side of the brain wall or 

 dividing the embryo into lateral halves. The latter form of mani- 

 pulation gave very satisfactory results in Salmo material but with 

 chick embryos this process was less satisfactory on account of the 

 hollow nature of the neural canal. 



Finally the embryos were sectioned in the ordinary way and the 

 sections studied in the light of the knowledge gained from the study 

 of the living and dissected specimens. Without this knowledge the 

 study of sections would certainly lead to confusion if not error. The 

 secondary divisions that frequently are present would eventually be 

 confused with the primary ones. 



The conventional way of studying cephalic metamerism has been 

 almost exclusively by the process of sections. Locr, '95, diverged 

 from this method by dissecting away the cephalic mesoderm in embryos 

 of the shark, and thus exposed the entire encephalon with its seg- 

 mental folds. I have used this method with gratifying results in 

 embryos of both the chick and the trout. By sweeping the neural 

 tube clean from all surrounding tissues, it is certainly true that with 

 properly reflected light, neural folds can be detected in this manner 

 that could not be observed in any other way. 



