411 



trunk." Nevertheless, in the young embryos of Squalus acanthias, 

 there seems to be a natural line of division. The medullary 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 does not begin very gradually, but, there is a difinite start- 

 ing point, 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 A A 1 in figs. 2, 3, 

 and 5. This line is, in Squalus acanthias, just in front of the point 

 where, subsequently, the vagus nerve begins. As before indicated 

 there are uniformly 11 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 11 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 bead. 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 assign- 

 able line of its union with the non-expanded trunk region, and there 

 are also in this form 10 or 11 segments included in the expanded 

 head part. 



Observations are now needed to determine whether these neural 

 segments are distinguishable in the earliest stages of other animals, 

 and if so, whether the number in the head region may be clearly 

 determined. 



Number of segments in fore- and hind-brain. 

 The observations recorded above have some bearing upon 

 the number of segments originally belonging to the different 

 parts of the brain. Waters, who especially studied the anterior 

 brain regions, came to the conclusion that there were primitively 2 in 

 the mid-brain, and 3 in the fore-brain, but, that this is not a primary 

 and genetic relation seems probable from what I have made out. 

 The segments, although they do not change in number in front of 



