118 ARTHUR DENDT. 



This is the parietal eye. It is further distinguished by the 

 presence of a distinct double circular outline, the inner circle 

 indicating the margin of the lens. The hinder of the two spots, 

 lying in the middle line or nearly so, represents the parietal 

 stalk, and, except that it sometimes appears dark in tbe centre 

 owing to its central cavity, it exhibits no further features of 

 interest. 



It is important now to notice the position of the parietal eye 

 and stalk with regard to the paraphysis, shown also in fig. 12. 

 It will be seen that whereas the former occupy the posterior 

 angle of the roof of the thalameucephalon, the latter appears 

 as a round white spot between the cerebral hemispheres, being 

 separated from the parietal eye by almost the entire length of 

 the thalamencephalon. The same relations are shown in fig. 9, 

 which represents somewhat diagrammatically a longitudinal 

 vertical section of the head at this stage, the parietal eye being 

 shown as though it lay in the middle line instead of slightly to 

 the left. 



The vesicle of the parietal eye is now somewhat deeper than 

 before, and the lens thicker (fig. 10). The ventral wall of the 

 vesicle is flattened against the roof of the thalamencephalon, 

 and its posterior wall against the anterior wall of the parietal 

 stalk. A few mesoblast cells have made their way in between 

 the lens and the superficial epiblast (epidermis), which appears 

 in the section figured to be slightly depressed just above 

 the parietal eye (probably owing to shrinkage in the course of 

 preparation). 



The cavity of the parietal stalk still opens into that of the 

 fore-brain, but the section passes a little to the left of the 

 opening. Its front wall is much flattened against the posterior 

 wall of the parietal eye, so that it appears to project vertically 

 upwards from the roof of the brain. 



Except for the thickening of the lens the wall of the parietal 

 eye at this stage has undergone no marked alteration. The 

 lens (fig. 19) is made up of more or less elongated columnar 

 cells with correspondingly elongated nuclei. These cells are 

 longest in the middle and much shorter towards the periphery 



