No. 3.] 



THE VERTEBRATE HEAD. 



531 



Torpedo is not so favorable for the study of the segments as 



Acanthias. The folds are much fainter in the former than in 



the latter, but the significant thing 



is, that the number in a given region 



in Torpedo corresponds to that in 



Acanthias. 



In the chick these segments may 

 be detected as soon as the neural 

 folds are established, before there is 

 any trace of protovertebrse. A small 

 number is visible in the blastoderm 

 of the twelfth hour of incubation just 

 as the head-folds are first outlined. 

 Their history has been carefully 

 traced by one of my students, Mr. 

 F. A. Hayner, and it agrees in all 

 essentia] features with the history of 

 the same segments in Acanthias. 

 Cut 7 shows the appearance of these 

 segments from surface view in a chick 

 embryo of about twenty-four hours 

 incubation. There are eleven seg- 

 ments in front of the first-formed 

 protovertebrae. The cell -arrange- 

 ment in the metameres, as shown in 

 sections, is the same as that described 

 by Orr in neuromeres of older stages 

 of the lizard (Anolis). 



From these supplementary obser- 

 vations it is clear that the occurrence 

 of primitive neural segments, as the 

 first definite expression of metamer- 

 ism, is not an isolated case in 

 Squalus. They occur in the same 

 very early period in all the other 

 forms examined, and in all of them 

 precede any division of the meso- 

 derm into protovertebrae. 



Cut 7. — Embryo of chick with four 

 mesodermic somites x about 45 

 diameters from a sketch by Fred. A. 

 Hayner. The neural folds are di- 

 vided throughout their entire length 

 into primitive metameric segments, 

 while there are but four mesodermic 

 somites found. 



