NEAL: nervous system in SQUALL'S ACANTHIAS. 189 



of the somites 2 to 5 (Katschenko, '88, Eabl, '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 (Eabl, '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 diflferentiation 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, '89). 



(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 

 pai'ticipates 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 1st, 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 (Piatt's) and 

 the 1st and 2d (van Wijhe's) mesoderm segments are now held : (1) that 

 they are serially homologous with trunk somites (van Wijhe, Piatt, 

 Hoffmann, Neal, Fiirbringer) ; (2) that they are abortive visceral pouches 

 (KupflFer, '88, Froriep, '92, SewertzofF, '95). The discussion, therefore, 

 turns upon the question whether these structures represent diverticula 

 (dorsal) of the mesoderm, or lateral diverticula from the alimentary canal. 



Miss Piatt ('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. 



