258 ELEMENTARY BIOLOGY. [CHAP. 



large pale granular cells, each possessing a 

 conspicuous clear round nucleus with a dis- 

 tinct nucleolus. 



/?. Tease out in glycerine a Gasserian ganglion 

 from a frog's head which has been preserved 

 in Miiller's fluid or chromic acid. Cells like 

 those described above will be found. 



y. Examine sections of spinal cord which have 

 been stained with carmine or hsematoxylin, 

 and note the large branched nucleated cells 

 in the grey matter especially towards the ven- 

 tral side of the cord. 



h. The retina. 



j. Sufficiently satisfactory specimens of this organ can 

 be obtained as follows. Take perfectly fresh eyes 

 from a frog, prick the corneas in two or three places, 

 and lay the eyes aside for three or four days in o'25 

 chromic .acid solution: then transfer them to alcohol 

 and keep them in it until wanted. 



a. Carefully cut open an eye preserved in the 

 above method and expose the retina: transfer 

 the latter to a glass slide, and with a razor chop 

 down on it so as to cut off a number of slices : 

 add glycerine, put on a cover, and examine 

 with a low power. Some of the bits will be 

 found thin enough for further examination. 



I. With the low power little can be seen but that 

 the retina is composed of a number of different 

 layers, some of which appear less opaque than 

 the others. 



