188 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



ci'epaucy 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 mesodei'mal 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 tlie 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 somatic value of his 

 somites : " (2) Dass die Lange der Somite sich im ganzen Korper gleich 

 verhalt. (3) Dass die obere Grenzlinie der Rumpfsomite ununterbrocheu 

 in diejenige der Kopfsomite tibergeht. (4) Dass die untere Grenze der 

 Somite sowohl im Kopfe als im Rumpfe nur Avenig unter der oberen 

 Grenze des Darmes liegt." The latter proof has been amplified by- 

 Kiliian ('91) from the evidence that the head somites are dorsal in 

 relation to chorda, dorsal aorta, and epibrancliial (medio-lateral) line. 



(5) Hoffmann ('94) and Miss Piatt ('97) have confirmed van Wijhe's 

 statement that the development of the somites begins in the neck region 

 and proceeds continuously both 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, 

 Kiliian, '91). To this Miss Piatt ('91) adds (7) the evidence that, as 

 in the case of the somatic musculature of the trunk, the muscles derived 

 from the " anterior," the first, the second, and the third somites (I'udi- 

 meutary 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, '96). 



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 



fiir die Richtigkeit seiner Ansicht " (Rabl, '89, p. 234). 



(4) The 1st somite is an exception to vau Wijhe's fourth argument 

 (Rabl, '89) ; moreover, the constrictions are never complete in the case 



