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 definitenesa 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 observ^ers, 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 YII 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 II.). 



