114 EARLY EMBRYOLOGY OF THE CHICK 



The original neuromeric enlargements persist longest in the 

 rhombencephalon. 



The three- vesicle condition of the brain is transitory. By 

 forty hours the division of the rhombencephalon into meten- 

 cephalon and myelencephalon is clearly indicated (Figs. 20, D 

 and 22). The division of the prosencephalon and the establish- 

 ment of the five-vesicle condition characteristic of the adult 

 brain, does not take place until somewhat later. 



In chicks of 55 hours (Figs. 34 and 35) the appearance of the 

 cranial flexure has resulted in the bending of the brain so that 

 the entire prosencephalon is displaced ventrad and then toward 

 the heart. At the same time the head of the embryo has under- 

 gone torsion and lies with its left side on the yolk. Although 

 flexion and torsion have thus completely changed the general 

 appearance of the brain as seen in entire embryos, the regions 

 already estabhshed in 40-hour chicks are still evident. The 

 prosencephalon has, however, become very noticeably enlarged 

 cephalic to the optic vesicles, and a slight constriction in its 

 dorsal wall indicates the beginning of the demarcation of the 

 telencephalic region from the diencephalic region. 



The Formation of the Telencephalic Vesicles. — By the end 

 of the third day the antero-lateral walls of the primary fore- 

 brain have been evaginated to form a pair of vesicles lying one 

 on either side of the mid-line (Figs. 39, 41, and 42, B). These 

 lateral evaginations are known as the telencephahc vesicles. 

 The openings through which their cavities are continuous with 

 the lumen of the median portion of the brain are later known 

 as the foramina of Monro. The telencephahc division of the 

 brain includes not only the two lateral vesicles but also the 

 median portion of the brain from which they arise. The 

 telocoele has therefore three divisions, a median, broadly con- 

 fluent posteriorly with the diocoele, and two lateral, connecting 

 with the median through the foramina of Monro (Fig. 42, C). 



Before the formation of the telencephalic vesicles the most 

 anterior part of the brain lay in the mid-line, but the rapid 

 growth of the telencephalic vesicles soon carries them anteriorly 

 beyond the median portion of the telocoele. The median ante- 

 rior wall of the telocoele which formerly was the most anterior 

 part of the brain, and which remains the most anterior part of 

 the brain lying in the mid-line is known as the lamina terminalis 



