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from the posterior boundary of this (7tk) somite to the posterior 
boundary of the encephalomere VZ (marked by an asterisk in 
Figure 1) after the cephalic plate has closed to form a tube. Con- 
sequently, I find no room in the cephalic plate for Locy’s “metamere” 11 
(encephalomere VII). Hither he has not traced the “neural segments 
in unbroken continuity until they become the neuromeres of other 
observers’, or an explanation of the discrepancy of our results is needed. 
3. Development of ““Neuromeres”. 
I divide neuromeres — i. e. segmental expansions of the neural 
tube — into encephalomeres, or expansions of the encephalon, and 
myelomeres, or expansions of the myelon, in accordance with the 
nomenclature proposed by Mc. CLurE (89). 
a. Encephalomeres. 
With the exception of Locy, ZIMMERMANN (91) is the only 
investigator who has studied the neuromeres in Selachii. For the 
purpose of comparison it is well to state his results here. He finds 
at first eight “primäre Abschnitte” in the encephalon, the first three 
of which exceed in size the last five. The first three are the Vorder- 
hirn, Mittelhirn and Hinterhirn, each of which he regards as a complex 
of encephalomeres, since they later subdivide into segments which 
dorsally are equally long and broad. The Vorderhirn divides into 
two encephalomeres, the Mittelhirn into three and the Hinterhirn 
into three. Thus, since the posterior five “primäre Abschnitte” do 
not further subdivide, there are in all thirteen “encephalomeres”. 
As a result of cephalic flexure some of the encephalomeres become 
wedge-shaped, but all are clearly separated from one another by 
constrictions. ZIMMERMANN’s preliminary paper, without figures, is, 
so far as I know, the only one that has yet appeared. 
Locy (95) finds eleven “neural segments” included in the 
cephalic plate, and to these are added in later stages segments from 
the trunk region (as is also stated by ZIMMERMANN), thus increasing 
the number of neural segments in the head to fourteen or fifteen. 
Of these the forebrain contains three and the midbrain, two. 
As has already been stated, I find six primary vesicles (en- 
cephalomeres) included in the region of the cephalic plate. The first 
three correspond with those called by Zimmermann, Vorderhirn, 
Mittelhirn and Hinterhirn. Behind these extend three smaller 
expansions of the neural tube similarly involving dorsal, lateral and 
ventral zones. I therefore regard these six primary expansions as 
morphologically comparable with one another. In later stages another 
