240 Journal of Comparative Neurology and Psychology. 



missure and longitudinally to enter the tectum they remain 

 "outside" of the central grey, that is near the original inner 

 surface of the ectoderm. When we take into consideration the 

 morphology of the optic stalk and the process of the brain ven- 

 tricle which it contains, the course of the optic tract fibers from 

 the retina to the brain is comparable in every respect to that of 

 the internal arcuate fibers from the cutaneous centers to the 

 tectum opticum. The migration of the optic stalk from the 

 dorsal to the ventral wall of the brain is now readily understood. 

 The retina must be carried out some distance from the brain 

 wall in order to reach the surface of the head. Since the centers 

 are carried out in the retina the stalk consists only of internal 

 arcuate fibers running through the ventral commissure. These 

 shift to the ventral wall as a matter of economy since in their 

 original position they must follow two sides of a triangle. 

 Furthermore, it will be noticed that if the retina is derived from 

 the dorsolateral wall of the brain its inner wall which becomes 

 thin is originally continuous with the choroid plexus and the 

 dorsal border of the outer wall of the retina may be regarded as 

 the dorsal border of the brain wall. In this respect the retina 

 is analogous to the acusticum which forms the dorsal part of 

 the wall of the oblongata, and the choroid plexus of the 

 fore-brain is analogous with that of the hind-brain. We could 

 say homologous instead of analogous except for the presence of 

 the equivalent of the peripheral ganglion in the retina. 



Finally, reference may be made again to the fact that a 

 frog is able to orient itself with reference to the source of light 

 which falls on the skin alone. It is even shown (102) that the 

 animal orients itself in the same way by means of the skin as by 

 means of the eyes, namely turns its head toward the light and 

 jumps toward it. This shows a close relation between the cen- 

 tral paths involved in motor reflexes in response to impulses 

 received from the skin and those received from the eyes. The 

 writer believes that the same paths were originally used for co- 

 ordinated reflexes by way of the tectum and the fact that these 

 relations are retained in an animal so highly developed as the 

 frog is a strong argument for the fundamental unity of the 



