DEVELOPMENT OP PETROMTZON FLUVIATILTS. 355 



first appeared. The lumen is at first circular in outline,, and 

 the walls of the canal of uniform thickness (fig. 11). Ulti- 

 mately in the region of the body the lumen becomes elon- 

 gated and slit like (fig. 24) ; in the anterior end the bimen 

 widens into the variously shaped cavities whicb form the 

 ventricles of the brain. The cells forming the walls of the 

 canal are primarily more or less cubical, but they soon 

 become spindle shaped, except those which form the roof 

 and the floor of the central canal. These are formed of 

 a single layer of short columnar cells. The canal is in 

 the youngest stages proportionately very much larger than in 

 the later ; its size is diminished and its form altered by the 

 thickenings which take place in different parts of the brain. 



The white matter first makes its appearance on the eighteenth 

 day as two thin bands, one on each side of the brain and 

 spinal cord (fig. 37). Later these unite in the ventral side 

 and form an anterior commissure. After the appearance of 

 the white matter the ganglion cells lose their spindle-shaped 

 outline and become again circular. 



The cranial flexure is very slight ; the anterior end of the 

 brain is, however, slightly bent down, and with it the anterior 

 end of the notochord (fig. 23). 



About the sixteenth day considerable changes take place in 

 the brain; from the anterior and ventro-lateral angles of the 

 fore-brain two diverticula are given off; these are the optic 

 vesicles (fig. 30). They continue to grow upwards and back- 

 wards till their blind end reaches a position behind and above 

 the anterior end of the notochord. 



At the blind end of the diverticulum a knob is formed by 

 the outer face proliferating cells, which form a multicellular 

 retinal layer. The posterior face later on developes pigment 

 in its cells. The lens is budded off from the inside of the 

 single layer of epidermis, and lies as a flattened mass of cells 

 close against the retinal layer (fig. 40). The stalk of the 

 primary vesicle becomes solid by its walls coalescing on all 

 sides, and forms the optic nerves. At their origin these nerves 

 form a commissure projecting into the cavity of the fore-brain 



