No. 3-] THE VERTEBRATE HEAD. 543 



8. Head and Trunk. 



It would be a great convenience to anatomists to have some 

 means of distinguishing between the head and trunk of very 

 young embryos. It is generally regarded as impossible, on 

 account of the lack of definite landmarks, to assign such a line 

 of division, in early stages, before the origin of the auditory 

 vesicle. As Sedgwick says, in the article referred to above, 

 "The term head here must be regarded as meaning the 

 anterior end of the body, for it is not possible in these young 

 embryos to distinguish the head from the trunk." Neverthe- 

 less, in the young embryos of Squalus acanthias, there seems 

 to be a natural line of division. The neural folds in this 

 animal run forwards with the margins nearly parallel to one 

 another, and then expand in front into the broad cephalic 

 plate. The expansion takes place rather abruptly, and it is 

 possible, in very young stages, to draw a line indicating where 

 the expanded part of the cephalic plate joins the non-expanded 

 part of the embryo. This line may be drawn without any 

 reference to the number of primitive segments that it will 

 cut off. The position of such a line is indicated by AA in 

 Fig. 26. This is, in Squalus acanthias, just in front of 

 the point where, subsequently, the vagus nerve begins. As 

 before indicated, there are uniformly eleven neural segments 

 in front of this line. Their number remains the same from 

 the earliest stages until the anatomical landmarks (auditory 

 vesicle and nerves) appear that enable us to determine the 

 limits of the head region. In this animal, we may identify 

 that part of the head that lies in front of the vagus nerve by 

 counting the first eleven neural segments. It will be merely 

 a question of agreeing upon the number of primitive segments 

 belonging to the vagus, to enable us to locate with definiteness 

 the hindermost limit of the head. Besides being of use in 

 other ways, this would enable us to say, even in the earliest 

 stages, what is head-mesoblast and what is trunk-mesoblast. 



It is interesting, in this connection, to notice that there is 

 in Amblystoma a similar greatly expanded cephalic part with 

 an assignable line of its union with the non-expanded trunk 



