THE SEGMENTATION OF THE HEAD. 



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the muscles of the trunk, and are called myotomes. In the 

 trunk-region these are the most markedly segmental of any 

 structures, and appear very early. Balfour pointed out that the 

 primitive body cavity extended into the head, and that this por- 

 tion also becomes divided in much the same way as that of the 

 trunk. So with him " head-cavities " are the test of cephalic 

 segmentation. As there is much similarity between his results 

 and those of his pupil Marshall, we may omit a discussion of 

 Balfour's conclusions and give those of Marshall in more detail, 

 merely saying that Balfour recognized only eight segments in 

 the head, with possibly one more. 



Marshall uses both head-cavities and nerves in determining 

 the segments, and he tried to bring gill slits into harmony with 

 these. He examined those nerves admittedly segmental, and 

 formulated the characteristics which a segmental nerve must 

 have. The results to which this brought him may be seen 

 from the table, but some explanations may be pardoned. 



