112 ME. W. G, EIDEWOOD OK THE HrOBRANCHrAti 



generally is a question wliicli cannot be regarded as definitely 

 settled. That they are only physiologically equivalent to the 

 mammalian thyrohyals, whicli they so closely resemble, is tolerably 

 certain. Ttie thyrohyals are derived from tlie branchial skeleton 

 in both, but from parts which are not homologous. The view 

 propounded more than seventy years ago by Meckel (27. p. 240) 

 and Cuvier (8. p. 397), that the thyrohyals of Anura represent 

 the fifth ceratobranchials of Elasmobrauch fishes and the lower 

 pharyngeal bones of Teleosteans, is well worthy of mature con- 

 sideration ; and it may be a not unimportant fact that the fifth 

 ceratobranchials are so much more strongly developed than the 

 preceding four in most Rays and Teleosteans. When, later, the 

 study of development disclosed the exceptional mode of origin of 

 the Amphibian thyrohyals, authors became silent concerning 

 their morphological value, and simply described them as out- 

 growths of the hypobranchial plate of the tadpole, behind the 

 fourth branchial arch. More recently Parker (35. p. 173, and 

 34), who was never at a loss for a working hypothesis, regarded 

 them as the fourth ceratobranchials, and in this he has been 

 followed by "Walter (40) and others. 



The fact of the thyrohyals arising so late and as outgrowths 

 from the hypobranchial plate, and not, like the branchial arches, 

 by differentiation of the cartilage, does not, however, appear to me 

 to militate seriously against our regarding them as the fifth 

 ceratobranchials. Their late development is certainly connected 

 in some degree with their relation to the larynx, which, like the 

 rest of the pulmonary respiratory system, does not attain any 

 considerable development until branchial respiration is waning. 

 Given five branchial arches, four of which are connected with 

 branchial respiration and the fifth with pulmonary respiration,, 

 and considering that respiration is at first branchial and later 

 pulmonary, it is in strict accord with such laws as we have been 

 able to formulate concerning the development of structures not 

 required for use simultaneously, that the fifth arch will not 

 develop until it is wanted ; and then, as is the case with most 

 organs the development of which is delayed, it will be formed 

 rapidly, and most probably in an exceptional manner. Similar 

 instances of accelerated and abbreviated development of organs, 

 the appearance of which has been delayed, may be seen in the 

 formation of the mesonephros of the frog from a blastema 

 instead of by peritoneal tubules, in consequence of the pro- 



