HISTORY OF THE EYE MUSCLES 439 



The evidence which has convinced the writer of the persistence 

 of the mandibular component of the external rectus muscle is 

 summarized in text figures 2 to 8 of this paper. In earlier papers 

 ('09, '14) the fact of its persistence was asserted, in agreement 

 with Dohrn ('04), but no evidence was presented. The facts 

 are as follows: In Squalus embryos of eighteen to twenty-four 

 millimeters the anlage of the external rectus muscle shows two 

 easily distinguishable elements, one (anterior) derived from the 

 myotome of the mandibular cavity and which is recognizable 

 as Miss Piatt's 'muscle E' the other (posterior) formed from 

 the myotome of the hyoid cavity {mytm. 3) The two elements 

 differ, not only in their staining properties, but the distinction 

 between the two may also be made out, as Miss Piatt has stated, 

 through the presence of a limiting membrane bounding the 

 myotome of the third (hyoid) head-cavity. As a result, however, 

 of the gradual disappearance of this membrane as development 

 goes on it becomes increasingly difficult to distinguish the two ele- 

 ments. The difficulty is further increased because of the forward 

 growth of myotome 3 (the hyoid element of the external rectus 

 anlage), which comes eventually to lie above the mandibular 

 element (mus. E.) Consequently, in embryos of 28 to 30 mm. 

 it is possible to distinguish the two elements clearly only in cross 

 sections of the muscle anlage. In still later stages the bound- 

 ing membrane disappears altogether. A slight difference how- 

 ever in the direction of the long axes of the muscle fibers of the 

 two elements makes it possible, even after the disappearance of 



Fig. 5 The external rectus muscle as it appears in a 24 mm. embryo in frontal 

 section. The muscle has thickened and elongated and the lumen of the third has 

 disappeared. Cross sections of embryos at this stage show the muscle anlage 

 much as in frontal sections (fig. 6). 



Figs. 7 and 8 The external rectus is cut lengthwise in frontal (horizontal) 

 sections of 26 mm. and 29 mm. Squalus embryos. In these stages it becomes in- 

 creasingly difficult to distinguish the two components of the muscle, especially 

 in frontal sections. The persistence of the mandibular component, however, is 

 undeniable. Id, 2d, dorsal moieties of the first and second head cavities; Iv, 2v 

 ventral moieties of the same; 3v, the third (hyoid) cavity; £^, mandibular com- 

 ponent (mus. E) of the external rectus muscle; ch. chorioid layer of the optic 

 vesicle; ep. epidermis; L, lens; x, dividing line between dorsal and ventral moie- 

 ties of the head-cavities. 



