DEVELOPMENT OF THE NERVOUS SYSTEM. 45 



of the neural plate. When the neural tube is formed these areas 

 at once bulge laterally. They seem upon casual observation, 

 for example in the chick, to in\-olve nearly the whole lateral wall 

 of the first two or three neuromeres. Careful examination shows 

 clearly, however, that in fishes, amphibia, birds and mammals, 

 these pouches belong strictly to the dorsal part of the brain, and 

 they probably belong to a single neuromere, the second. The 

 pouches grow out farther, retaining a portion of the brain ventricle, 

 and finally come to be constricted next 'to the brain so that they 

 remain connected with the brain wall only by a narrow stalk. 

 The pouch is now known as the optic vesicle and the stalk as the 

 optic stalk. Figures i8 to 20 show the optic vesicle in various 

 stages of growth and Figures 21 and 22 represent it in transverse 

 section. As the vesicle is formed it is crowded between the brain 

 and ectoderm so that it is flattened. The flattening continues 

 until the cavity becomes a narrow cleft and the vesicle becomes 

 somewhat saucer shaped. The constriction from the brain takes 

 place from above downward so that the stalk comes to be attached 

 near the ventral border of the vesicle. As a result of the same 

 process the attachment of the stalk to the brain has shifted ventrad 

 to reach the position afterward occupied by the optic chiasma. 

 In the meantime the two walls of the optic vesicle begin to be 

 differentiated, the outer wall growing thicker to form the retina 

 and the inner wall remaining thin. Finally the portion of the 

 brain ventricle which was carried out into the optic vesicle is 

 obliterated but in many lower vertebrates a vestige of the cavity 

 is retained in the stalk. This lumen connects with the preoptic 

 recess, a depression of the floor of the third ventricle in front of 

 the optic chiasma. 



Histogenesis. — The development of the microscopic structure, 

 the histogenesis, of the brain, nerves and sense organs is going 

 on at the same time with the development of the form of the brain. 

 The neural tube consists at first of two incomplete layers of cells 

 irregularly arranged (Figs. 13, 14), among which are some columnar 

 cells which extend through the whole thickness of the wall. A 

 Httle later the columnar cells become radially arranged around the 

 lumen of the tube (Figs. 14, 15), and the shorter cells lie between 



