STRUCTURE OF THIRTY-THREE HOUR CHICKS 65 



telencephalon and diencephalon. the mesencephalon remains 

 undivided, and the rhombencephalon divides to form the 

 metencephalon and myelencephalon. 



The division of the prosencephalon into telencephalon and 

 diencephalon is not completed until a much later stage of 

 development, but the median enlargement at this stage ex- 

 tending anterior to the level of the optic vesicles indicates where 

 the telencephalon will be established (Fig. 20, D). The optic 

 vesicles and that part of the prosencephalon lying between them 

 go into the diencephalon. 



The mesencephalon, as stated above, undergoes no subdivi- 

 sion. The original mesencephalic region of the three- vesicle 

 brain gives rise to the mesencephalon of the adult. This region 

 of the brain does not undergo any marked differentiation until 

 relatively late in development. 



At this stage the division of the rhombencephalon is clearly 

 marked (Fig. 20, D and Fig. 22). The two most anterior 

 neuromeres of the original rhombencephalon form the meten- 

 cephalon and the posterior four neuromeres are incorporated 

 in the myelencephalon. 



The Auditory Pits. As is the case with the central nervous 

 system, the organs of special sense arise early in development. 

 The appearance of the optic vesicles which later become the 

 sensory part of the eyes has already been noted. The first 

 indication of the formation of the sensory part of the ear 

 becomes evident at about 35 hours of incubation. At this age 

 a pair of thickenings termed the auditory placodes arise in the 

 superficial ectoderm of the head. They are situated on the 

 dorso-lateral surface opposite the most posterior inter-neuro- 

 meric constriction of the myelencephalon. By 38 hours of 

 incubation (Fig. 22) the auditory- placodes have become 

 depressed below the general level of the ectoderm and form 

 the walls of a pair of cavities, the auditory pits. When first 

 formed the walls of the auditory pits are directly continuous 

 with the superficial ectoderm, and their cavities are widely open 

 to the outside. In later stages the openings into the pits 

 become narrowed and finally closed so that the pits become 

 vesicles lying between the superficial ectoderm and the myelen- 

 cephalon. As yet they have no connection with the central 

 nervous system. 



