382 
plate from the region posterior to it marks the posterior boundary 
of the auditory invagination. When the cephalic plate closes, it in- 
cludes six primary encephalic vesicles, or, as I shall call them, en- 
cephalomeres (Fig. 1). 
Fig. 1. A parasagittal section of an embryo of S. acanthias at a stage just 
following the closure of the neural tube, showing the six primary vesicles (encephalomeres), 
included within the limits of the cephalic plate. I—VI, encephalomeres; 1—3, 5—7 
VAN WIJHE’S somites; sac. vse. 1, sac. vsc, 2, visceral pouches; gn. trg., ganglionic 
Anlage of trigeminus; *, posterior boundary of cephalic plate. 
The sixth, or last of these, probably corresponds with Locy’s 
10 neural, or auditory, segment, since this is the segment opposite 
which the auditory invagination takes place. The seventh primary 
encephalic vesicle or encephalomere, which corresponds with Locy’s 
eleventh (glossopharyngeus) “neural segment”, and is, he says, included 
in the cephalic plate, I find becomes differentiated (at a somewhat 
later stage than the one represented in Figure 1) out of the region 
behind the cephalic plate. My determination of the limit of the 
cephalic plate has been made by careful measurements in a large 
number of individuals. I have taken as a fixed point the posterior 
boundary of van WisHr’s 7h somite (the first somite innervated by 
a spinal motor root), the identity of the somite being determined by 
measuring in camera-projection images the distance of this somite 
from the cleft in the mesoderm which separates van WiJHE’s 2nd from 
his 3rd somite. The distance from the posterior limit of van W1JHE’s 
7th somite to the posterior limit of the cephalic plate has thus been 
found to correspond (a very slight and constant difference ascribable 
to the increase in length of the embryo is perceptible) to the distance 
