32 H. LISSER 



The liyoid hone and styloid process. These bodies are easily 

 made out existing as pure condensed mesenchyma. .The styloid 

 l)rocess is more advanced than the greater cornu of the hyoid at 

 this time. 



The epiglottis. This cartilage can be discerned, much flattened 

 in comparison with the adult type. It shows as a beginning area 

 of condensation. 



THE MUSCULATURE 



The eight nmscles are first visible at this period of development, 

 not all as individual muscle masses however, although it is true 

 that one or two of the muscles can be clearly differentiated from 

 the rest. It is curious to note that the laryngeal musculature 

 shows more advanced differentiation than the pharyngeal constric- 

 tors, as shown by clearer outline and more extensive fibrillation. ^ 

 The intrinsic oesophageal musculature however is farther devel- 

 oped. The sphincter formation, to which attention has been called 

 by so many writers, can be recognized at this stage, and in a few 

 sections there is some tendency to continuity between the fibres 

 of the outer pharyngeal ring and the inner laryngeal one. Prob- 

 ably too much stress has been laid on this structure, perhaps by 

 reason of the fact that an analogous muscle has been found in 

 lower animals. At any rate, the muscles of the larynx differen- 

 tiate themselves much earlier than has been previously believed. 

 For instance, the crico artyaenoideus posterior (fig. 1) is unmistak- 

 ably isolated at this stage, well defined of good size, and abundant 

 fibrillation ; nor does it include any other laryngeal muscle. There 

 is also a muscle mass with some fibrillation, not so large, but plainly 

 evident, which apparently includes the crico aryteanoideus later- 

 alis and thyreo arytaenoideus (fig. 2), principallj^ the former. It 

 is placed on the lateral surface of the cricoid cartilage. The inter- 

 arytaenoideus; if it exists at all, does not appear as one muscle. 

 In the position where one would expect to find it, there is con- 

 tinuity of the larynx and pharynx lumina. But just at the point 

 where these join there are muscle fibres on either side, but which 



