140 LARVAL OCELLI AND THE PARIETAL EYE. 



unknown function. It is difficult to understand how any one familiar with visual 

 organs, could fail to recognize in the parietal eye of Petromyzon, or Hatteria, or 

 Lacerta, a visual organ of some kind. The pigment, lens, retinal cells, and nerves. 

 are unmistakably parts of an organ that served at one time as an eye. whatever 

 its function may be now. 



The conflicting accounts of the parietal eye are due in part to the various 

 conditions in which it appears in different groups of vertebrates, but mainly 

 to a fundamental misconception of the ground plan of the organ and how it 

 happens to get inside the brain. 



It has not been clearly recognized that the parietal eye is a paired organ 

 arising originally outside and beyond the boundaries of the brain; that it contains 

 several distinct sensory placodes; that there is a fundamental distinction between 

 the sensory placodes and the non-sensory epithelial tube that connects them with 

 the brain; and it has been very difficult to eliminate the idea that the paraphysis is 

 an eye or a part of one, or that it produces some part of the parietal eye. 



From our new point of view, the parietal eye of vertebrates is a most signifi- 

 cant and illuminating organ. The best insight into its meaning may be obtained 

 by studying its structure and development in the lamprey. 



Petromyzon. My observations on the development of the eyes of this 

 animal in the main, agree with those of Sterzi, especially in regard to the nature 

 of the early epyphysial outgrowth. 



The Parietal Eye Vesicle. In larvae about o mm. long, a single median 

 parietal eye tube is seen just in front of the superior commissure. The dilated 

 end of this tube appears to divide into two lobes, the larger one lying outside of, 

 and a little in front of the other. The inner one lies somewhat to the left of the 

 median line, the outer one to the right; the displacement, however, is not enough 

 to indicate that the two sacs are right and left lobes of a single pair. 



I have no material representing the stages between 6 mm. and 30 mm. 

 larvae, and I do not know just what takes place at this critical period, but the next 

 following stages seem to indicate clearly enough that, meantime, the inner sac has 

 become separated from the main tube, giving rise to an endo-parietal, or "para- 

 pineal" eye; while the outer sac remains connected with the primitive eye tube, 

 giving rise to the "pineal" or ecto-parietal eye. (Fig. 100, ec.p.e.and en. 

 p.e.) The floor of each sac is now divided by a deep longitudinal groove, consist- 

 ing of undifferentiated epithelium, into two symmetrically placed, concave discs, 

 each disc probably representing a retinal placode. (Fig. 101.) Both sacs develop 

 a small amount of brownish pigment, which is, however, masked by a large 

 quantity of the characteristic white granules. The entire organ, when seen with 

 the naked eye, is a glistening white spot that looks precisely like the endo-parietal 

 eye of Limulus. In both Limulus and Petromyzon, the granules are soluble in 

 weak acid. 



The outer eye sac presents the most characteristic retinal structure. In 

 six inch ammoccetes, the retinas consist of a layer of sensory cells, each bearing 



