EMBRYOLOGY , 231 



vesicle which is derived from the ectoderm, and in this portion the ear 

 bones are much later developed out of mesoderm. The connection of 

 this first pouch with the digestive system either remains open or after 

 closing re-opens again, in either case forming the Eustachian tube which 

 connects the middle ear with the pharynx. 



The outer ear develops first as an invagination from the outside. 

 The pit or canal thus formed approaches the middle ear, but always 

 remains separated from it by a membrane, the ear drum. The shell-like 

 external ears of animals that possess them are erected out of mesoderm 

 with, of course, an ectodermal covering in the epidermis of the skin. 

 The outer ear is not shown in the illustrations (Fig. 182). 



Olfactory Organ. — The olfactory organ, like the ear, is at first a patch 

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

 ectoderm invaginates (Fig. 182, off), 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 ectodermal part of the mouth. An opening is 

 subsequently formed at this point of contact, and the nostril is thus con- 

 nected 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 olfac- 

 tory nerve. The nasal chamber in many vertebrates becomes curiously 

 altered in form, and receives the ducts of various glands. 



The development of the few organs described in the foregoing pages 

 will suffice to illustrate the kinds of processes involved in organogeny, 

 especially in structures arising from the outer and inner germ layers. 

 Organs and tissues derived from the mesoderm, such as muscles, bones, 

 heart, blood vessels, kidneys and their ducts, are as a rule much less 

 sharply defined in their early stages than are those of the ectoderm and 

 endoderm. A description of their early development is therefore omitted. 



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

 large measure the same for different members of the same group of animals, 

 as described above for a few organs of the vertebrates, some members of 

 a group may undergo an unusual series of changes. Metamorphosis 

 is one such series of developmental processes. Animals that metamor- 

 phose are born or hatched with one, or several, or many organs which 

 they will not possess as adults, and they may retain these organs for a 

 considerable period after birth or hatching. Or the young animal may 

 lack organs that will be developed before it becomes adult. The process 

 of losing the larval organs and of gaining the missing adult organs is 

 called uK^tamorphosis. 



Metamorphosis in Invertebrates. — Metamorphosis is common among 

 insects, in many groups of which the young stages are wholly unlike the 

 adult. In butterflies, beetles, bees and flies, the egg hatches into a 



