438 



H. V. NEAL 



'^—Z'^ira.o'bl. sup.) 

 1*^ (m.rect. sup. ) 



(ra.rect. int.) 



g4mm. Frontal. ^^V^ 



ZOmm. Sag. lv.(TD.obl. inf . ) 

 2V("E?) ^.. ^^ r-h. 



SOmm. Cross 



Figs. 2 to 8 Semidiagrammatic camera drawings of certain stages in the onto- 

 genesis of the external rectus muscle as seen in Squalus embryos of 18 mm. to 29 

 mm. length. The mandibular component of the external rectus muscle (Miss 

 Piatt's muscle E) is shown in black with unshaded nuclei, while the hyoid com- 

 ponent is drawn unshaded with black nuclei. Head-cavities 1, 2, and 3 are 

 shown in topographic relation to the eye-ball only in figure 2. The anlage of the 

 external rectus alone is shown in the remaining figures (3 to 8). 



Fig. 2 The double-bimeric-origin of the external rectus muscle. The two 

 components are in intimate contact from this stage on. The figure is drawn from 

 parasagittal sections of a 20 mm. Squalus embryo. The division of the head 

 cavities to form the six eye muscles has already begun. 



Fig. 3 The anlage of the external rectus muscle in a cross section of an 18 mm. 

 embryo of Squalus. The mandibular component (Mus. E. ) appears as a differentia- 

 tion of the mandibular cavity, the lateral wall of which has already begun to dis- 

 integrate into mesenchyme. The multiplication of embryonic muscle cells in 

 the median wall of the third head cavity has begun to obliterate the lumen of that 

 cavity. In a 20 mm. embryo the conditions remain essentially unchanged 

 (fig. 4). 



