226 EDWARD A. BOYDEN 



through, at a coijisiderably older stage than that represented by 

 the human embryos under discussion. Unless, therefore, some- 

 thing similar can be found in the branchial region of the lower 

 vertebrates, it hardly seems as if the structure described by 

 Grosser could be regarded as an internal rudimentary gill. With 

 more likelihood it n%ay be compared with those outpocketings 

 in the preauditory region of the pharynx of the chick which 

 Kastschenko as doubtfully considered ' ' vermutliche rudimen- 

 tjire Schlundtaschen." 



Apparently filaments do not occur in mammals. The exten- 

 sive series of mammalian embryos which are available in the 

 Harvard Collection have been searched in ^ vain for traces of fila- 

 ments comparable with those already described for the Saurop- 

 sida. Reviewing the phylogeny of the branchial system of ver- 

 tebrates in the light of these facts, it would seem that the gills 

 of the lowest vertebrates have given place to functionless homo- 

 logues in the Sauropsida and that with the further reduction 

 which the branchial system has undergone in mammals all 

 traces of even vestigial filaments have disappeared. 



CONCLUSIONS 



In the Sauropsida the development of the branchial region is 

 characterized by the formation and relatively late persistence of 

 a band of tissue across the ventral surface of the neck, which has 

 been derived from the ventral union of the hyoid arches, and 

 which may be known from its resemblance to the development 

 of the gill cover of certain fishes and amphibians as the opercular 

 fold or plica opercularis (Kiemendeckelwulst of German authors). 



On the lateral margins of this operculum, after it has grown 

 backward to enclose at least a potential peribranchial chamber, 

 filamentous outgrowths may be observed on its under side, which 

 in reptiles have a very transitory existence but which in the chick 

 undergo a relatively extensive and prolonged development. 



On account of the filamentous character of these outgrowths, 

 their origin from the branchial arches (the epithelium of which 

 Ekman has shown to possess a certain specificity for gill-forma- 

 tion in the Anura), and their constant relation to the operculum 



