Johnston, Morphology of the Head. 235 



sufficient to enable the animal to orient itself with reference to 

 the source of light. The skin of many other animals also has 

 been shown to be sensitive to light. It may be added that the 

 retina is sensitive to pressure, sending an impulse to the brain 

 which is there interpreted as light. 



There are certain resemblances between the eyes and the 

 general cutaneous ganglia in the mode of their development. 

 In Petromyzon {y6) the eye appears first as a solid outgrowth of 

 the wall of the brain and the cells have the appearance of mi- 

 grating cells. In this there is a similarity to the mode of form- 

 ation of the trigeminus ganglion (j6, figs. 4 and 7). In sel- 

 achians, on the other hand, both the optic vesicle and the gan- 

 glia of the cranial nerves appear as hollow buds or outpocketings 

 of the brain wall. This has been mentioned by Hoffmann (59) 

 and a comparison drawn between the eye vesicle and the gan- 

 glia in this regard. His figures suggest very strongly that 

 something more than an accidental resemblance obtains. In 

 amphibia (29) the eyes appear first as pigment spots toward the 

 margins of the cephalic plate where a certain rearrangement of 

 cells takes place long before the vesicles begin to bulge out. 



From their position in amphibia one would think that the 

 eyes were dorso lateral structures belonging to the same neuro- 

 mere as that to which the stalk is attached after the vesicle is 

 formed. This appears more clearly in the selachians figured by 

 LocY (125). In all vertebrates the vesicle includes a large part 

 of the lateral wall of the fore-brain and owing to the constriction 

 taking place from above downward the stalk comes to have a 

 ventral position. The mode of formation of the optic stalk 

 strongly suggests that the vesicle was not originally a ventral 

 structure, as it is usually regarded, but rather a dorsal one whose 

 attachment to the brain has moved downward or been constricted 

 downward, as happens in the ontogeny. In Lepidosiren (72), 

 indeed, this process is recapitulated in much fuller form than in 

 most vertebrates. The solid optic process appears at first ex- 

 tending laterally from the dorsal wall of the brain. A cavity 

 appears in it and becomes continuous with the cavity of the 

 tore-brain. "And at the same time, by differential growth, the 



