150 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
It is well known that in embryos of all classes of Vertebrates the 
central nervous system shows a segmentatiog which consists in a series 
of constrictions and dilatations extending throughout the length of the 
neural tube, giving to it a beaded appearance. In the trunk the nerves 
have a definite relation to the segments of the spinal cord, the “myelo- 
meres,” as I shall call them, adopting the term introduced by McClure 
(89), and it is believed that the cranial nerves also have definite re- 
lations to the segments of the brain, the “ encephalomeres,” although 
their relations are less clear. Even if we believe with Ahlborn ("84") 
and Froriep (94) that the nervous system is segmented in adaptation 
to associated segmental structures, it is not a priori improbable that the 
number of primitive segments in the Vertebrate may be shown by the 
number of neural segments, for in some Invertebrate embryos segmental 
cephalic ganglia appear even when most other traces of mesodermal 
segments and related sense organs have (it is believed) disappeared. 
In view of the present discrepancy between the results based upon the 
study of neuromerism and those based upon the study of mesomerism, it 
devolves upon one who attempts to elucidate the question of cephalic 
segmentation in Vertebrates by using the segments of the central ner- 
vous system as criteria, to show the comparability of encephalomeres 
with myelomeres, not only structurally, but also in relation to ner- 
vous outgrowths, and to those divisions of the mesoderm on which 
the segmentation of the motor nerves ultimately depends. The inter- 
dependence of motor nerve and muscle has seemed so evident that 
morphologists have not hesitated to make the number of cranial nerves 
conform with the number of somites previously determined by them. 
Yet the majority of investigators of the segments of the encephalon have 
failed to take into consideration the relation of these to the segments of 
the mesoderm, and consequently we find in the literature upon neuro- 
merism a diversity of opinion such as we have learned to expect in 
results based upon insufficient knowledge. 
SUMMARY OF RESULTS OF FORMER INVESTIGATIONS ON THE 
SEGMENTATION OF THE EINCRPHALON. 
The results of former investigators concerning the number of enceph- 
alomeres and their nerve relations may be summarized in the form of 
two tables. Table I. shows the number of segments as determined by 
previous investigators, as well ás their relation to the primary vesicles 
of the brain. The total number of segments has been given in the 
+ cases where it has been stated by the observer. The observations of 
