SARGENT: THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 181 



from the upper to the lower margin of the fold, they give a somewhat 

 radiated appearauce to this region because of the sharp flexure of the 

 wall. This region of the brain thus has early in development a char- 

 acteristic appearance (Plate 4, Figs. 24, 25), which is remarkably 

 constant throughout life in all the lower vertebrates. 



(1) Optic Reflex Cells. The first difi'erentiation in the neuroblasts 

 takes place in Amia just before hatching, in the anterior portion of the 

 roof of the third ventricle. In the region which later becomes the anterior 

 part of the optic tectum, some of the neuroblasts have increased in size, 

 have become more spherical in form and. more definite in outline, and 

 take stains somewhat more deeply than the suiTounding neuroblasts 

 (Plate 4, Fig. 23, ch ojA. rfx.). These cells are concentrated for the most 

 part on either side of the median plane. A few scattering cells extend 

 posteriorly and laterally through the tectum. There are from twenty to 

 thirty of these cells at their first appearauce, but they rapidly increase 

 in number by the differentiation of additional cells. 



In Amia the first day after hatching the number of cells is from 

 30 to 60. They are spherical and from 9 to 1-i micra in diameter, 

 thus being conspicuous among the surrounding neuroblasts (Fig. 24, 

 n'hh), whose outlines are less definite and whose diameters are less 

 than half as great. The tectum in the median plane is so thin at 

 this stage that a single cell often occupies its whole thickness. The 

 protoplasm of the cell stains deeply, and the nucleus is large and clear, 

 its diameter being somewhat more than- half that of the cell. The 

 single nucleolus stains deeply and is about two micra in diameter. 

 The nucleus is central, not eccentric as in later stages. During the 

 first day the axons develop from tlie cells as extremely fine processes. 

 They first appear always on the side of the cell nearest the ventricle, 

 growing directly toward it (Fig. 24, ax.). About the end of the first, 

 or the beginning of the second day, these axons penetrate into the 

 ventricle, passing between the epithelial or ependymal cells Avhich line 

 the roof of the ventricle. 



In larvae from two to three days after hatching, the cells have in- 

 creased in size and number. The number, somewhat variable at all 

 stages (as shown in the table, p. 185), is now from 60 to 100. The 

 average cell has now a diameter of 14 micra, two or three times that of 

 the surrounding neuroblasts. The nucleus is large, 8 micra in diameter, 

 and eccentrically placed opposite the side of the cell from which the axis- 

 cylinder emerges. The tectum at its anterior end is still so thin that 

 the cells lie in a single layer. The axis-cylinder, a delicate fibril, comes 



