440 



H. V. NEAL 



the limiting membrane, to distinguish the two in cross sections. 

 We see therefore that what disappears is not the mandibular 

 element but the limiting membrane bounding the hyoid element, 

 making it increasingly difficult and finally impossible to dis- 

 tinguish the two. Of the disintegration or degeneration of the 

 muscle cells of 'mu-scle E' there is not the slightest evidence. 

 On the contrary, in the stages during which degeneration has 

 been said to occur, the embryonic muscle cells of both elements 

 of the external rectus muscle undergo similar progressive dif- 



Fig. 9 A camera drawing of a parasagittal section of a 5 mm. Squalus embryo 

 (Series ICC 1-2-1) showing VanWijhe's somites 1 to 6. They are seen to be dor- 

 sal segments of the mesoderm and their segmentation to be independent of the 

 visceral arches. A, the anterior cavity; 1, 2, 3, 4, 5, 6, VanWijhe's somites 1 to 

 6; F.B., forebrain vesicle; g.s.' , first gill-pouch; M.B. midbrain vesicle; sp. spirac- 

 ular pouch. 



ferentiation as elongated spindle-shaped muscle fibers. In both, 

 myofibrillae are visible in Squalus embryos of forty-five milli- 

 meters and transverse striae in embryos of one hundred milU- 

 meters. 



Concerning the ontogenesis of the other eye-muscles there 

 is no difference of opinion. All later observers, including the 

 writer ('98), have confirmed the results of Marshall ('81). It 

 is quite unnecessary, therefore, in this paper to repeat the de- 

 scription of what is so well known. In the light of the evidence 

 presented in this paper, however, it seems necessary to revise 



