146 THE INFANCY OF ANIMALS 



In the majority of living birds, it must be mentioned, this 

 sheath is formed of a continuous piece of horn, but in 

 some, as in the ostrich tribe, the gannets and cormorants 

 and their kin, and the petrels, it is made up. of several 

 distinct plates. That this compound sheath was once 

 a common characteristic is shown by the fact that in the 

 embryos of many species, as for example in the common 

 heron, these separate plates still make a fleeting appearance, 

 and traces thereof are to be found in the beaks of many 

 adult birds of various kinds. 



The second feature to which reference has been made 

 is one of a more striking character, since it intimately 

 concerns so vital a process as that of breathing. Briefly, 

 the aperture of the nostrils which pierces this sheath in 

 certain adult birds is closed, and the process of this closure 

 is a gradual one which can be studied in the embryonic 

 and nestling stages. 



I had the good fortune to make this interesting dis- 

 covery some years ago, while examining a recently dead 

 cormorant. Herein, the most careful research failed to 

 reveal any external nostrils, though a trace thereof seemed 

 lying in a groove near the base of the beak. On forcing 

 the plates of the compound sheath apart and raising 

 them from the underlying skeleton I found a short plug 

 of horny tissue lying to the inner side of the groove which 

 I suspected originally marked the position of the external 

 nostril, and this plug had plainly been withdrawn from a 

 small aperture in the underlying bony skeleton. Passing 

 a bristle down this aperture it at once emerged from the 

 posterior nostrils in the roof of the mouth, thus reveahng 

 a most unexpected state of affairs. 



Turning, when opportunity afforded, to the examination 

 of embryos and very young nestlings, I found that external 



