NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 189 
of the somites 2 to 5 (Katschenko, ’88, Rabl, ’89), so that it is impos- 
sible to state the position of their lower boundary with reference to the 
dorsal wall of the alimentary canal (Rabl, ’89). 
(5) The development of the “ head cavities” is discontinuous with 
that of the trunk somites (Rabl, ’89, Kupffer, 93). While the develop- 
ment of the pre-otic segments takes place later than that of the trunk 
somites, the differentiation of mesenchyma takes place much earlier 
in the head than in the trunk. This conflicts with the law, that in the 
Anlagen of serially homologous organs the older the Anlage the earlier 
the histological differentiation (Rabl, 789). 
(6) There never appears in the case of the pre-otic segments a differ- 
entiation into myotome and sclerotome (Rabl, ’89, p. 235). 
(7) While the musculature of the trunk and occipital somites arises 
exclusively from the median wall of the somite, the musculature of the 
pre-otic segments has its origin in greater part from the lateral, and in 
smaller part from the posterior wall of the so called somites.! Moreover, 
while only a distinct and sharply defined portion of the trunk somites 
proliferates mesenchyma, the entire median wall of the pre-otic segments 
participates in the formation of mesenchyma (Rabl, ’89). 
(8) The topographic relations of the dorsal nerves in later stages are 
different in head and trunk. In the head the nerves grow laterad to the 
somites, while in the trunk they grow mediad to them (Rabl, ’89). 
Special arguments, in addition to the general ones stated above, con- 
cerning the nature of the anterior, the Ist, and the 2d mesoderm 
segments have been made, because of their marked peculiarities in de- 
velopment, structure, and relations, and of their important bearing upon 
the question of the morphology of the eye muscles, It will therefore be 
necessary to state these also, 
Two chief opinions concerning the nature of the anterior (Platt's) and 
the Ist and 2d (van Wijhe's) mesoderm segments are now held : (1) that 
they are serially homologous with trunk somites (van Wijhe, Platt, 
Hoffmann, Neal, Fürbringer) ; (2) that they are abortive visceral pouches 
(Kupffer, '88, Froriep, 92, Sewertzoff, 95). The discussion, therefore, 
tuins upon the question whether these structures represent diverticula 
(dorsal) of the mesoderm, or lateral diverticula from the alimentary canal. 
Miss Platt (’91, ’91*) argues for the somatic value of the anterior 
somite (cavity) as follows:— (1) In position, independence, and time of 
origin this cavity resembles the following ones. (2) Many cells from its 
1 Balfour ('81) holds that both median and lateral walls of the trunk somites 
form the lateral trunk musculature. 
