188 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
crepancy in the results of investigators of the mesomeric, as well as of the 
neuromeric, segmentation most certainly justifies Rabl’s (89) complaint 
of the hasty way in which investigators have given mesodermal segments 
somatic value. In no question of morphology to-day is conservative 
judgment more needed. Before stating my own evidence I will briefly 
summarize the arguments advanced by previous investigators for and 
against the somatic value of the mesodermal segments of the head. 
(1) In addition to the evidence first stated by Marshall ('81), that the 
dorsal mesoderm of the head of Selachian embryos undergoes a segmenta- 
tion independent of the segmentation of the visceral arches, van Wijhe 
(82, p. 4) uses the following arguments for the somativ value of his 
somites : “(2) Dass die Länge der Somite sich im ganzen Körper gleich 
verhält. (3) Dass die obere Grenzlinie der Rumpfsomite ununterbrochen 
in diejenige der Kopfsomite übergeht. (4) Dass die untere Grenze der 
Somite sowohl im Kopfe als im Rumpfe nur wenig unter der oberen 
Grenze des Darmes liegt.” The latter proof has been amplified by 
Killian (91) from the evidence that the head somites are dorsal in 
relation to chorda, dorsal aorta, and epibranchial (medio-lateral) line. 
(5) Hoffmann (94) and Miss Platt ('97) have confirmed van Wijhe’s 
statement that the development of the somites begins in the neck region 
and proceeds continuously botlt posteriorly and anteriorly. Furthermore 
(6) the same constituent parts, viz, myotome and sclerotome, may be 
distinguished in the head as well as in the trunk somites (van Wijhe, ’82, 
Killian, ’91). To this Miss Platt (91) adds (7) the evidence that, as 
in the case of the somatic musculature of the trunk, the muscles der: ved 
from the “anterior,” the first, the second, and the third somites (ruaı- 
mentary in the case of the anterior and somewhat modified in the case of 
the first somite) first appear in the median wall of these somites. Finally 
(8) there is a correspondence of the neuromeres and mesodermic segments 
throughout the entire length of the neural tube (Neal, 796). 
The following are the arguments advanced in opposition to the somatic 
value of the mesodermal segments of the head. 
(1) The divisions of the mesoderm of the head are due to the me- 
chanical influence of the neighboring parts, chiefly that of the visceral 
pouches (Kastschenko, ’88). 
(2) The divisions are irregular in size (Kastschenko, 88, Rabl, ’89). 
(3) In van Wijhe’s third proof there is “nicht die Spur eines Beweises 
für die Richtigkeit seiner Ansicht” (Rabl, "89, p. 234). 
(4) The 1st somite is an exception to van Wijhe’s fourth argument 
(Rabl, 89) ; moreover, the constrictions are never complete in the case 
