240 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
lature). The motor fibres of nerves V, VII, IX, and X belong to the 
former, and nerves III, TV, and VI to the latter class. 
While it is possible, as has been stated, to establish a numerical cor- 
respondence of encephalomeres and somites, the nerve relations are not 
so clear. We find, for example, that encephalomeres II, III, and VII 
are connected by ventral (motor) nerves with somites (van Wijhe’s) 
1, 2, and 3. Such evidence of a want of segmental correspondence 
would seem at first sight to render untenable the assumption that 
encephalomeres have the same segmental value as myelomeres. We 
have already seen that these two classes of neuromeres have structurally 
little in common. Moreover, a want of correspondence of encephalomeres 
and visceral arches is shown by the fact that the dorsal motor fibres 
which are connected with encephalomeres IIL and V innervate two suc- 
‘cessive visceral arches. In view of this discrepancy in the segmental 
relations of encephalomeres and nerves, can we regard the former of 
segmental value? Do they afford evidence in support of the assumption 
that a Vertebrate head segment is comparable, i. e. homologous, with a 
trunk segment? Before expressing my own opinion in regard to the 
answer to this question I will briefly review the interpretations given 
by previous investigators. Two antithetic views concerning the neu- 
romeres have been given, viz. (1) that they are not of segmental or 
phylogenetic value, and (2) that they are of phylog.  '. value. 
VII. Segmental Value of Hindbrain Neuromeres. 
a. NoN—PHYLOGENETIC INTERPRETATION. 
In 1877 Mihalkovics, speaking of the foldings in the medulla of birds 
and mammals, expressed the opinion that the want of correlation be- 
tween these structures and the nerves and visceral arches seems to favor 
the view that they are of mechanical origin, i. e. formed by the bending 
and shoving of the neural tube as it rapidly grows in a confined space. 
This view seems strengthened by the consideration that the ventral wall 
of the neural tube of chick embryos is, in early stages, markedly folded 
into segments, irregular in size and inconstant in appearance, and that 
these folds in the head region are visibly exaggerated by certain fixing 
agents which result in shrinking the embryo. Balfour, who with Foster 
(’74) had been the first to express the opinion that these structures 
were of phylogenetic significance, afterwards (81) said that it is uncer- 
tain whether they have any morphological significance. In 1892 
