436 . H. V. NEAL 



is, How many head-cavities participate in the formation of the 

 external rectus muscle? 



Miss Piatt ('91) was the first to observe that in Elasmobranch 

 embryos a muscle which she calls 'muscle E' arises in such in- 

 timate connection with the external rectus muscle that for some 

 time she believed that the mandibular cavity "took part with 

 the third in the formation of the external rectus muscle." Sub- 

 sequent observation has confirmed this observation of Miss 

 Piatt. There is disagreement only concerning the supposition 

 that the mandibular component ('muscle E') degenerates. Does 

 this 'muscle E' — the mandibular component of the embryonic 

 external rectus muscle — degenerate? 



In answering this question Miss Piatt compared two stages 

 ('91, figs. 5 and 6, pi. 5) which correspond quite closely with 

 text figures 6 and 8 of this paper. She says (p. 86) : 



This section (sect. 5) also shows the peculiar relation existing between 

 the external rectus muscle, now forming in the third head cavity,- and 

 the mandibular muscle, mus. E. If this cross section be compared 

 with a similar section through an older embryo, represented in sec. 6, 

 it will be seen that were the cells of the mandibular muscle (mus. E.) 

 to fuse with those of the third head cavity (ext. rec.) at the stage repre- 

 sented by sec. 5, the resulting muscle would closely resemble in shape 

 the external rectus of sec. 6. I am convinced, however, that such a 

 fusion does not take place, for the limiting wall of the third head cavity 

 can be traced until the muscle here formed comes to occupy the entire 

 place once occupied (sec. 5) by the cells of the two muscles (ext. rec. 

 and mus. E.). The cells of the mandibular muscle (mus. E.) gradually 

 yield their place to those of the third head cavity and are ultimately 

 lost in the general mesoderm. Thus a muscle, the rudiment of which 

 appeared in the walls of the mandibular cavity prior to the origin of 

 any of the eye muscles, completely disappears, although in the embryo 

 of 22 mm. it is still relatively large as compared with the eye muscles. 



In the description of a 27 mm. embryo of Squalus she goes 

 on to say (p. 87) : "The cells of the rudimentary muscle (mus. E.), 

 so closely related to the external rectus, are now indistinguish- 

 able from the surrounding mesoderm, and the superior oblique 

 muscle represents all that is left of the walls of the dorsal part 

 of the mandibular cavity." Of the histological changes mani- 

 fested by degenerating muscle cells Miss Piatt presents no 

 evidence whatever. 



