THE NERVOUS SYSTEM OF AMPHIBIA 275 



into the space between the skin and the retina, practically into 

 contact with the lens both dorsally and ventrally. The pigment 

 layer of the retina has now become highly pigmented, so that 

 most of its nuclei are obscured from view. Mitosis is still going 

 on in the middle of the outer layer of the retina, but it appears 

 to be more abundant towards the margins. In the inner layer 

 of the retina there now occur many ganglion cells, particularly 

 in the middle region. Good silver impregnations show that the 

 axones of these cells form an optic nerve which extends along 

 the optic stalk into the brain, where they enter the most caudal 

 portion of the chiasma ridge. The form and position of these 

 cells are illustrated in figure 71. The axone of this cell passes 

 into very close relation with the internal limiting membrane of 

 the retina, where it mingles with other axones. Nothing can be 

 made out concerning dendritic processes of these cells. They 

 appear still to be unii)olar. 



According to favorable siher impregnations of both A. punc- 

 tatum and A. microstomum Cope, these optic fibers form a 

 chiasma which is in intmiate relation with postoptic commissure 

 at this time. The optic fibers, being more deply impregnated 

 than the other fibers of the commissure, may be followed clearly 

 across the middle line of the brain. They appear to be a con- 

 stant feature of the brain of the early swimming embryo, though 

 they do not stand out clearly in stained preparation as tho\- do 

 in silver impregnations. 



In the same preparations which demonstrate the ganglion cells 

 and their fibers clearly nothing of a nervous nature can be seen 

 deeper in the retina. There are some round nuclei which sug- 

 gest neuroblasts destined to become bipolar cells; but the layer 

 of rods and cones seems veiy embryonic, since, as mentioned 

 above, mitosis is still going on in its central region and no well 

 defined cellular organization appears there. 



5. The olfactory organ and nerve 



In the non-motile stage the olfactory organ consists of a greatly 

 thickened patch of ectoderm which has externally a very slight 

 concavity. It touches the brain only lightly in its dorso-caudal 



