208 MYOTOMES OF THE HEAD. 



rather a later period. Their walls however persist, and become 

 more columnar. In PL xiv. fig. 13 6, pp., is represented the 

 cavity in the last arch but one, at a period when the cavity in 

 the mandibular arch has become greatly reduced. It occupies 

 the same position on the outer side of the aortic trunk of its 

 arch as does the cavity in the mandibular arch (PL Xiv. 

 fig. 7, 2pp), In Torpedo embryos the head-cavity is much 

 smaller, and atrophies earlier than in the embryos of Pristiurus 

 and Scy Ilium. 



It has been shewn that, with the exception of the most 

 anterior, the divisions of the body-cavity in the head become 

 atrophied, not so however their tvalls. The cells forming these 

 become elongated, and by stage N become distinctly developed 

 into muscles. Their exact history I have not followed in its 

 details, but they almost unquestionably become the musculus 

 contrictor superficial is and musculus interbranchialis^ ; and pro- 

 bably also musculus levator mandibuli and other muscles of 

 the front part of the head. 



The most anterior cavity close to the eye remains unaltered 

 much longer than the remaining cavities, and its two halves 

 are still in communication at the close of stage L. I have not 

 yet succeeded in tracing the subsequent fate of its walls, hut 

 think it probable that they develojje into the muscles of the eye. 

 The morphological importance of the sections of the body- 

 cavity in the head cannot be over-estimated, and the fact that 

 the walls become developed into the muscular system of the 

 head renders it almost certain that we must regard them as 

 equivalent to the muscle-plates of the body, which originally con- 

 tain, equally with those of the head, sections of the body-cavity. 

 If this determination is correct, there can be no doubt that they 

 ought to serve as valuable guides to the number of segments 

 which have coalesced to form the head. This point is, how- 

 ever, discussed in a subsequent section. 



General mesoblast of the head. — In stage G no mesoblast is 

 present in the head, except that which forms the walls of the 

 head-cavity. 



During stage H a few cells of undifferentiated connective 



1 Vide Vetter, Die Kicmen wid Kiefermusculatur d. Fische. Jenaische Zcit- 

 schrift, Vol. vii. 



