60 H. LISSER 



is necessary to suppose that the interlacement, which is surelj^ 

 present according to my findings in an embryo of 20 mm., should 

 disappear and reappear again at 48 mm. ; for surely no very radical 

 changes occur in development, as regards the larynx, after 20 mm. 



M. inter arytaenoideus. The fibres of this muscle show up beau- 

 tifully in cross sections, and are unmistakably present in abund- 

 ance in this stage. The muscle is well defined and quite thick. 

 Figs. 30 and 36 give an idea of it, both in section and reconstruction. 

 I hardly see why there should have been any difficulty in separat- 

 ing it from the cricoarytaenoideus posterior or thyreoarytaenoi- 

 deus as was reported by Kanthack. The fibres run in a different 

 direction and reconstruction shows it quite as sharply defined as 

 in the adult. 



M. aryepiglotticus. (figs. 35 and 36.) There is some doubt in 

 mind as to whether the fibres which seem to make up this muscle 

 can be clearly isolated from the interarytaenoideus ; whether per- 

 haps, the adult appearance is not due to the later development 

 upward of the epiglottica. Primarily no doubt, these two mus- 

 cles originate as. one mass and at this stage cannot as yet be defin- 

 itely isolated. The fibres which make up the aryepiglotticus 

 seem to be continuous with the interarytaenoideus, merely having 

 extended upward along the lateral surface of the epiglottis. 



M. thyreoepiglotticus. At this stage and in this embryo, there 

 seems to be an indication of this muscle as brought out in the re- 

 construction, figs. 35 and 36. It seems reasonable to assume 

 that this muscle is originally unrecognizable from the thyreoary- 

 taenoideus, and is again dependent for its differentiation upon the 

 development upward of the epiglottica, which draws these fibres 

 away from the other muscle mass. This has apparently occurred 

 at this stage. 



The constrictors. The esophageal musculature is very well 

 advanced by this time, and by nature of the attachments to the 

 superior cornu of the thyreoid and the greater cornu of the hyoid 

 bone, a middle and inferior constrictor mass may be recognized. 

 Fibrillation- is extensive. 



^ Folio. viiiR I lie ilea of Dr. Lewis, I have used the word fibrillation, not with the 

 view of indicatinfi; fibrillae. but with reference to the definite direction of the 

 muscle fibres. 



