NEAL! NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 163 
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 meso- 
blast and what is trunk mesoblast.” * 
I cannot see that Locy’s determination of the limits of the cephalic 
plate helps us at all in the determination of the boundary of head and 
trunk. This boundary, as he states, has still to be determined. To fix 
the limits of head-mesoderm by a direct study of the mesoderm itself is 
quite as easy as to determine its boundary by the still hypothetical pos- 
terior boundary of the vagus region. According to Locy, the posterior 
limit of the cephalic plate separates neither what is pre-otic from what 
is post-otic, nor head from trunk. 
My own observations on this point differ fundamentally from those of 
Locy, since according to my determination the line which separates the 
expanded cephalic plate from the region posterior to it marks the pos- 
terior boundary of the auditory invagination, This is of value, in so far 
as it enables us to distinguish those two regions — which on other 
grounds have always been held to be distinct —in stages earlier than 
was formerly possible. The posterior boundary of the cephalic plate is 
a clearly marked point at a stage before the neural folds begin to be 
raised dorsally, and it is situated just behind the region of greatest 
ventral flexure of the cephalic plate (marked by an arrow in Fig. 3, 
Plate 1). This point may be traced into later stages, until the neural 
plate is transformed into a closed tube, when it is seen that it corre- 
sponds exactly with the hinder boundary of the hindbrain neuromere 
numbered VI in my figures (Locy’s 10th “neural segment”) ; opposite 
this neuromere, as has been stated by many observers, lies in early 
stages the centre of the auditory invagination. The thickened auditory 
epithelium extends anterior and posterior to this neuromere ; but it is 
opposite this neuromere that the first invagination to form an enclosed 
capsule takes place (see Plate 3, Figs. 15 and 16). In later stages the 
ear capsule shifts backward, so that its centre comes to lie opposite the 
hindbrain neuromere numbered VII in my figures, which, as may be in- 
ferred from the statement above, lies in —or rather is afterwards differ- 
entiated from — the region behind the cephalic plate. I have been able 
to determine with certainty that the posterior limit of the cephalic plate 
1 Locy finds that in later stages segments are added to the occipital region from 
the region of the trunk (see Tables I. and IL). 
