MYOTOMES OF THE HEAD. 431 



like those in its anterior part, though at rather a later period. 

 Their walls however persist, and become more columnar. ' In 

 PI. 15, fig. 13 b,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 (PI. 15, fig. 7, 2pp}. In Torpedo embryos 

 the head-cavity is much smaller, and atrophies earlier than in 

 the embryos of Pristiurus and Scyllium. 



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 walls. 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 

 constrictor superficialis and musculus interbranchialis 1 ; 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, but think 

 it probable that they develope into the mitscles 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 contain, 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, however, discussed in a sub- 

 sequent 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 Kiemen und Kiefermusculatur d. Fische." Jenaische Zeit- 

 schrift, Vol. vn. 



