384 
Similiar expansion appears behind encephalomere VI (encephalomere 
VII of my figures). These seven encephalomeres are well shown 
in a chick embryo of about thirty-three hours incubation (Fig. 2). 
* 
N ro ———— = 
ee 
a) 
“) 
= Gn acs-fae. 
a 
Fig. 2. A parasagittal section of a chick embryo with 13—14 somites, showing 
the encephalomeres. As in S. acanthias, the cells of the acustico-facialis Anlage are 
proliferated from the fifth encephalomere (V). J—VZIJI, encephalomeres; gn. acs -fac., 
ganglionic Anlage of the acustico-facialis ; *, posterior boundary of auditory invagination. 
I am unable to regard the secondary subdivisions of the first 
three encephalomeres as serially homologous with the typical hind- 
brain neuromeres, exemplified in encephalomeres IV, V and VI. 
Not only does the lateness of the differentiation of the “secundäre 
Abschnitte” offer a strong reason for doubting the primitive nature 
of such segments, but also the difficulty must be met of comparing 
such dorsal expansions as the “secundäre Vorderhirn” and “Zwischen- 
hirnblase” with such ventral structures as the midbrain and hindbrain 
expansions. Again, if we may have secondary encephalomeres, why 
may we not have tertiary as well? Finally, if we are to show the 
serial homology of encephalomeres with myelomeres, it is necessary 
to show that they have the same relations to motor nerves and somites. 
This has not been done by those who have used the secondary sub- 
divisions of the primary brain vesicles as criteria of metameres. 
Upon the closure of the cephalic plate encephalomeres IV, V and VI 
are seen in frontal sections to have their lateral zones locally thick- 
ened. At slightly later stages similiar local thickenings appear in 
the posterior part of encephalomere ZII and in the region of encephalo- 
mere VII (see Fig. 3). On the ventral and dorsal zones these 
thickenings do not exist, the encephalomeres being simple expansions 
of the neural tube, as shown in Figure 4, which represents a frontal 
