Lect. IV.] THE FEEBLE-FACED PANGOLIX. 107 



spoken of tliis change in tlie non-placental types (those 

 whose embryos are born in a very immature state), 

 but as the process is greatly misunderstood by most 

 anatomists, I will again describe it in this type, one 

 of the best in which to trace it. 



The deep and superficial parts that form the jaws in 

 all types, except the Mammalia, are so arranged that 

 the upper part of the first visceral arch forms the upper 

 jaw, whilst the lower segment swings on it as the 

 mandible, or lower jaw. When the first gill-opening, 

 namely, that between the top of the first and the top of 

 the second arch, instead of developing a rudimentary 

 gill, is formed into a pouch opening inwards (a rudi- 

 mentary drum cavity), then the topmost segment of 

 the second, or hyoid, arch fits itself into a smal 

 opening in the wall of the ear-capsule on the side of the 

 " vestibule." I am now using terms of human anatomy. 

 The opening formed by the dehiscence or splitting of the 

 wall of the capsule is the fenestra ovalis ; the operculum 

 or lid, which is taken from the top of the tongue-arch 

 or '^ hyoidj' is the stapes (or stirrup). 



This is not stirrup-shaped until we get some height in 

 the mammalian class ; in many lower forms, and in all 

 the oviparous Vertebrata, it is a little column, with a 

 broad, flat, oval base, a columella. This may be 

 composed of from one to five segments, but the 

 homology is the same throughout ; it is merely the 

 swinging-piece, or top segment of the second, or hyoid, 



