EMBRYOLOGY OF THE CHICK 323 



two ends. Beneath the anterior end, and behind the heart, there 

 gradually accumulates the cellular basis of the liver. 



The commencement of the development of the organ of hearing 

 is by a superficial depression of the cephalic blastema to meet the 

 process from the encephalon, which forms the acoustic nerve. 

 The lining of the depression becomes, on closure of the slit, the 

 proper tunic of the labjnrinth. 



The vesicle of the labyrinth swells into four dilatations, of which 

 three are ampullar and the fourth cochlear. The ampullar dilata- 

 tions 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-caUed cutaneous follicle becomes a 

 circumscribed sac or vesicle, in which the changes and the develop- 

 ment next proceed, converting the vesicle into an acoustic labyrinth 

 or into an eyeball. In each case neural elements of two vertebrae 

 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 vertebrae, 

 and the other between the frontal and the nasal viertebrae. 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. 



