214 PRINCIPLES OF ANIMAL BIOLOGY 



the eye, from the central nervous system. A patch of ectoderm on each 

 side of the head region thickens and then invaginates (Fig. 184A), 

 producing a pear-shaped vesicle. The vesicle is pinched off from the 

 ectoderm and comes to lie within. It changes its shape, producing the 

 characteristic semicircular canals and the (sometimes) coiled body of 

 the inner division of the ear. Nerve cells growing out from the ganglion 

 of the eighth (auditory) nerve join the vesicle with the brain. 



The middle ear, which contains the bones of the ear, is derived at 

 least in part from the first gill pouch (Fig. 184A, B, C). In the distal 

 portion of this pouch the ear bones are developed out of mesoderm, while 

 its connection with the digestive system, as already stated, forms the 

 Eustachian tube which connects the middle ear with the pharynx. 



The olfactory organ, like the ear, is at first a patch of thickened ecto- 

 derm on each side of the head far to the front. This ectoderm invaginates 

 (Fig. 184, olf), but unlike the ear the pit thus formed does not close; it 

 remains open to the outside as the nostril. The pit enlarges and protrudes 

 inward to meet the front end of the digestive tract just behind the ecto- 

 dermal part of the mouth. An opening is subsequently formed at this 

 point of contact, and the nostril is thus connected with the deeper portion 

 of the mouth cavity. Only certain parts of the ectoderm that forms the 

 olfactory cavity become sensory. From these parts nerve processes 

 grow toward the brain, thus forming the olfactory nerve. 



Metamorphosis. — Besides the usual course of development, which is 

 in large measure the same for all vertebrate animals, some members of 

 that group undergo an additional series of changes called metamorphosis. 

 Animals that metamorphose are born or hatched with one or more 

 organs which they will not possess as adults, or lacking organs that will 

 be developed before they become adult. It is the process of losing the 

 larval organs and of gaining the missing adult organs which is called 

 metamorphosis. 



The transformation of a tadpole into a frog (Fig. 187) or toad is the 

 classical example. The readily visible changes are the degeneration 

 of the so-called "sucker" or attaching organ beneath the head; the 

 development of the legs; and the absorption of the tail, the material of 

 which is probably used elsewhere for growth. The external gills, hidden 

 under a fold of skin called the operculum, disappear early, to be replaced 

 by internal gills which are developed on the endodermal lining of the gill 

 slits. The internal gills are lost later, and their function served by lungs, 

 which have all the while been developing. The jaws are provided with a 

 horny armature, serving as teeth, but these are shed and the mouth 

 increases greatly in size. The intestine, from the early tadpole stage a 

 \(iiy long and much coiled tube, is greatly shortened. 



Other kinds of animals undergo metamorphosis, notably among the 



