No. 3-] THE VERTEBRATE HEAD. 537 



incipient neural folds. He expressly states that the segmental 

 divisions do not extend beyond the plate into the neural ridges. 

 The latter, he says, become segmented later ; only, when in 

 the process of closing of the neural groove, they are brought 

 directly over the originally segmented plate, and are made to 

 feel the effect of the underlying segmented mesoblast. The 

 number of segments which he detects in the neural plate at 

 this period is very small. He finds only four in the greatly 

 expanded anterior part. 



My observations (already given on p. 530), differ from his in 

 a fundamental way: showing, in Amblystoma, Rana palustris 

 and the newt, the presence of unmistakable segments, in the 

 earliest formed neural ridges, and not less than ten pairs in 

 the broadly expanded head-plate. 



In Squalus acanthias, also, it is the incipient neural folds 

 that are the most plainly segmented, and in this animal 

 they are so situated that they cannot possibly come under 

 the influence of the mesoblast in any mechanical way. It 

 will be remembered from descriptions of Figs. 27 to 30 and 

 from sections of the same (cuts 3, 5, 6) that the neural folds 

 are formed as wing-like expansions, extending laterally beyond 

 the body. The mesoblast does not accompany these outfold- 

 ings of the epiblast, and when they best exhibit the segmental 

 divisions no mesoblast enters into them. 



As for the rest, Froriep gives figures of sections to show that 

 the mesoblast is at this very early period actually divided into 

 somites that correspond to the external segments. His figures 

 certainly show segmental divisions, but being wood-cuts they 

 are not entirely satisfactory when the question arises whether 

 they are really protovertebrae. Froriep interprets them as 

 such and closes his article as follows : " Die Gliederung des 

 Wirbeltierkorpers ist urspriinglich an das mittlere Keimblatt 

 gebunden ; wo sich als Produkte des ausseren Keimblattes 

 segmentale Anordnungen finden, sind dieselben durch Anpas- 

 sung an die Metamerie des Mesoblastes sekundar entstanden." 



The internal condition figured by Froriep is interesting 

 when compared with that in Squalus acanthias at corresponding 

 stages. I have found in that animal an undulated condition of 



