THE LATER DEVELOPMENT OF THE CHICK 303 



In earlier stages neuromeres have made a brief, transitory 

 appearance in the fore- and mid-brains as well. Thus in the 

 embryo of about twenty-three hours (4 pairs of somites), Hill 

 has distinguished eleven neuromeres, all told. Of these three 

 are in the region of the future fore- brain: these disappear 

 about the twenty-sixth hour (7 pairs of somites). The mid- 

 brain includes only two neuromeres, which remain distin- 

 guishable until about the thirty-fifth hour (14 pairs of somites). 

 The six neuromeres of the hind-brain are the best marked, and 

 are visible for several days (Fig. 122). 



The next important advance consists in the transverse con- 

 striction of the primary fore- and hind-brains each into two 

 sections, making in all five regions in the brain. The pri- 

 mary fore-brain, or prosencephalon, is divided into the anterior 

 telencephalon, or secondary fore-brain, and the posterior 

 diencephalon, or between-brain. Then following the un- 

 divided mid-brain, or mesencephalon, come the two divisions 

 of the primary hind-brain, or rhombencephalon; the anterior 

 of these is known as the metencephalon, the posterior as the 

 myelencephalon. The myelencephalon continues directly into 

 the spinal cord. 



We may now proceed to describe the principal structures 

 arising in connection with each of the secondary divisions of 

 the brain. 



The telencephalon includes only the first neuromere. This 

 portion of the brain, it will be recalled, is directed backward 

 beneath the fore-gut, its morphologically anterior end actually 

 being directed postero-dorsally (Fig. 123). The telencephalon 

 expands vertically and the optic vesicles remain connected 

 with its ventral side. As the optic vesicles push out toward 

 the surface of the head, their connections with the brain 

 become narrowed as the optic stalks. The cavities of the 

 vesicles remain continuous, through the stalks, with the 

 cavity of the telencephalon (Fig. 127), and a median depression 

 in the floor of the telencephalon, between the openings of the 

 optic stalks, becomes well marked as the recessus options (Fig. 

 123). This optic recess, and a short stretch of the brain- wall 



