No. 3-] THE VERTEBRATE HEAD. 523 



should say from my observations that, more frequently, it is 

 not so widely expanded. It always settles down in Squalus 

 acanthias, to occupy the position first indicated, and, subse- 

 quently, it is shifted backwards. The topography of the head 

 region is similar to what it was in Fig. 32. The chief 

 differences to note are the further differentiation of the 

 branchial arches and clefts ; we may now distinguish the 

 position of the future first visceral cleft, and, faintly outlined, 

 the boundaries of branchial arches. 



The segment marked 8 serves as an important landmark in 

 all subsequent changes that affect the segments. It is seated 

 above a depressed region in which the first visceral cleft 

 subsequently appears, and, during all the time the segments 

 are distinguishable from the outside, it has no nerve root. 



Only a few words of description will be needed to enable us 

 to follow the history of the metameres through the later 

 stages. In Fig. 34, the auditory vesicle {aii) has been formed 

 and the first visceral cleft has broken through. The anterior 

 metameres lying in front of the one marked 6 are no longer 

 distinguishable from surface observation. The lines of neuro- 

 meres have been brought into contact in the median plane by the 

 closing of the neural groove, but they are soon forced apart by 

 the growth of the dorsal wall of the neural tube. We have 

 now reached the stage, approximately, in which these neuro- 

 meres have been described by previous writers, — that is, the 

 stage just after the appearance of the auditory vesicle, but it 

 is to be remembered that Kupffer and Froriep have noted a 

 form of segmentation in very early stages of Amphibia affecting 

 the neural plate, but not the neural ridges (see p. 529). 



In Fig. 35 some characteristic changes are to be noted ; the 

 auditory vesicle has shifted backwards till it occupies a position 

 opposite the eleventh metamere ; the metameres are being 

 forced apart laterally by the growth of the dorsal wall of the 

 hind-brain. The eighth metamere still serves as a landmark ; 

 there are now two clearly marked metameres in front of it and 

 three behind it. The only metameres discernible from surface 

 view are those belonging to the hind-brain. The mid-brain is 

 considerably increased in expanse, and the first accessory 



