The vesicle of the labyrinth swells into four dilatations, of which three are ampullar and the fourth cochlear. The _ampullar dilatations_ extend into very slender canals, at first almost in the same plane, by which they are brought into mutual communication. As the canals expand and elongate, they assume their characteristic relative positions as external, superior, and posterior, the posterior end of the external canal being extended beneath the posterior canal. The cochlear dilatation curves as it elongates. An inner layer becomes distinct from the common membrane and forms the _acoustic lamina_. As in the development of the ear, so in the development of the eye, the production of the nerve process from the cerebral center is the first step; the infolding of the superficial blastema to meet the nerve is the next. The so-called _cutaneous follicle_ becomes a circumscribed sac or vesicle, in which the changes and the development next proceed, converting the vesicle into an acoustic labyrinth or into an eyeball. In each case neural elements of two vertebræ become modified to lodge and to protect the sense organs, forming respectively the recess called _otocrane_ and that called the _orbit_. The one is located between the occipital and the parietal vertebræ, and the other between the frontal and the nasal vertebræ. The part of the outer blastemal layer of the head which sinks to meet the process from the mesencephalic dilatation, rapidly changes its follicular into a vesicular state. The vesicle thus formed elongates, bending around the cell mass in which the _crystalline lens_ is formed, and the meeting of the two ends results in forming the choroid fissure at the lower part of the eyeball. The _mesencephalic process_, or optic nerve, expands at the posterior of the circular sac, and, in the course of mutation into eyeball, lines its posterior part with a layer which becomes the _retina_. The transparent layer covering the forepart of that sac and the inclosed lens is formed into the _cornea_. Other layers of the sac are formed into the _choroid_, the _ciliary processes_, the _iris_, and the _pecten_. Of the appendages of the eye the membrana nictitans is the first to develop. Then develop the lower lid and, last, the upper lid. After the development of the essential organs of sense, the skin is developed. Modifications of the skin form the outer ear and the eyelids. Then are formed the maxillary arch, the hyoidean arch, and the scapular arch.