DEVELOPEMENT OF BIRDS. 261 



grows it turns upon its left side, exhibiting a profile view ; it then 

 indents the yolk, and finally almost divides it into two portions. 



The formation of the digestive tube and glands closely follows 

 the course described in Vol. I. pp. 604, 605, 606. 



The embryo of the bird is that which best admits the observa- 

 tion of the commencement of the developement of the organ of 

 hearing by a superficial depression of the cephalic blastema, fig. 

 135, f, to meet the process from the epencephalon, ib. e, which 

 forms the acoustic nerve. The lining of the depression becomes, 

 on the closure of the slit, the proper tunic of the labyrinth.^ 



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

 which three are * ampullar,' and the fourth ^ cochlear : ' the am- 

 pullar dilatations extend into very slender 

 canals, at first almost in the same plane, by 

 which they are brought into mutual com- 

 munication : as the canals expand and elon- 

 gate, they assume their characteristic relative 

 positions as external, superior, posterior : the 

 hinder end of the external canal being ex- 

 tended beneath the posterior canaL The 

 cochlear dilatation curves as it elongates : an 

 inner layer becomes distinct from the common Fore part of embryo chick 



, , p ^-1 i- 1 • second day. 



membrane, and lorms the acoustic lamma. 



As in the developement of the eye, the production of the nerve- 

 process from the cerebral centre 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 developements next proceed, converting 

 the vesicle into ^ acoustic labyrinth' or * eyeball.' In each case 

 neural elements of two vertebrae become modified to lodge and 

 protect the sense-organs, forming respectively the recesses called 

 *otocrane' and ''orbit,' the one between the occipital and parietal 

 vertebrae, the other between the frontal and nasal vertebra?. 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 elongates, bending 

 round the cell-mass in which the crystalline lens is formed (as in 

 the Fish, Voh I. fig. 423), and by the meeting of the two ends, the 

 ' choroid fissure,' at the lower part of the eyeball, figs. 134, 135, 

 results. The mesencephalic process, or * optic nerve,' expands at 



* In XX. pi. Ixx. fig. 3, embryo of the Goose at the thirtieth hour of incubation, the 

 open state of the acoustic sac is erroneously described as ' meatus: ' 1)ut the sac be- 

 comes closed, and the tympanum and its passage are later developements. 



