5 1 2 LOCV. [Vol. XI. 



same region. I think, also, there is a way to bring Froriep's 

 observations into reconciliation with my own (see p. 529). 



At all events, it has been understood from the work of pre- 

 vious observers that the neural segments arise after the neural 

 groove is closed, or while it is in process of closing. Waters 

 (•92) carries the idea throughout his paper that the metameric 

 segmentation arises relatively late, especially in the Teleosts, 

 where he was unable to find any traces of this segmentation 

 earlier than the ninth day of development, after the auditory pit 

 is formed. Orr and McClure do not in every case state ages, 

 but from their figures and the text I understand that they 

 have not detected this segmentation in very young stages. 

 Miss Piatt mentions the fact that these segments sometimes 

 occur in the chick while the groove is open. Rabl mentions 

 them as being especially clear from the fiftieth to the ninetieth 

 hour of incubation in the chicken. 



I have been fortunate enough to find these neural segments 

 in a number of animal forms in extremely young stages, and 

 in Squalus acanthias, to trace them coherently onward into 

 the later stages. In this form, the division of the embryo 

 into segments takes place before the neural groove is formed, 

 and, before any protovertebrae have made their appearance, the 

 metameres not only extend the whole length of the embryo, 

 but they are continued for some distance into the embryonic 

 rim. They occur under such conditions in this animal that 

 they cannot be interpreted as depending on mesoblastic seg- 

 mentation. In Amblystoma and the newt (Diemyctylus) the 

 metameric segmentation is present in the rudiments of the 

 neural folds, just after their first appearance and during their 

 period of broadest expansion. In living chick embryos of 

 about the twentieth hour of incubation they can be made out 

 with clearness along the walls of the beginning neural folds. 

 It is only in Squalus acanthias, however, that I have traced 

 the complete history of these neuromeric segments. 



