208 EDWARD A. BOYDEN 



operculum or gill cover, which grows back over the sinus behind 

 it, enclosing the third and fourth arches with their filaments in a 

 branchial chamber. In the later stages of the chick this arch 

 undertakes a similar backward extension, the significance of 

 which was first appreciated by Rathke, the pioneer discoverer of 

 gill clefts in amniote embryos.- In the wingless bird Apteryx 

 (Parker), and in Struthio (Nassonow), the opercular fold to which 

 this backward extension gives rise is said to be a very conspicuous 

 object, much more so than in the other amniotes, although in all 

 vertebrates, the hyoid is larger than any of the other arches.' 

 Thus the persistence of this peculiar development of the hyoid 

 arch through the whole vertebrate series is almost as striking as 

 the persistence of the arches and diverticula themselves. In the 

 light of this fact the presence of filamentous structures behind 

 the operculum takes on added significance. 



Returning to the discussion of figure 5, a careful examination 

 of the third arch reveals a small hillock or mound just behind 

 the point of the hyoid, — between the point and the third ectoder- 

 mal groove. In fresh specimens observed under salt solution, 

 this mound is distinctly whiter than the surrounding tissue and 

 is the first external indication of the appearance of filaments. 

 Serial sections of a slightly older stage than that referred to in 



- "Am vierten unci fiinften Tage der Bebriitung kommen auf jeder Seite in 

 der Substanz des Halses drei aufeinander folgende fast linsenformige Hohlen 

 zum Vorschein, deren jede nach aussen und innen geoffnet ist. Die aussere 

 Mlindung der vordersten Hohle wird tibrigens von einem Theile, der ahnlich dem 

 Kiemendeckel der Fische ist, verdecktJ' ('25) 



In a much later publication ('61) the same author carries the analogy still 

 farther. "Von dem zweiten Schlundbogen, in welchem sich ein Zungenbeinhorn 

 ausbilden soil, wachst bald darauf, nachdem sich die vordeste Schlundspalte 

 geschlossen hat, ein klappenartiger Fortsatz hervor, der die zweite Schlundspalte 

 bedeckt und als eine Andeutung der Membrana branchiostega der Grotenfische 

 betrachtet Averden darf." 



' Parker on Apteryx: "The backward extension of the hyoidean fold visible 

 in the previous stages has increased so as to form a true operculum, which com- 

 pletely covers the third cleft, so that it is invisible in an external view. The 

 fourth cleft .... lies immediately behind the operculum, and is very 

 probably only exposed by the shrinking of the latter The reten- 

 tion of so obviously ain{)hibian a character .... appears to be a charac- 

 ter of very considerable morphological interest." 



