230 HOWARD AVERS. [Vol. IV. 



pose of image perception, — simple light-perception sufficing for 



all its needs. 



As Hatschek has shown, the pigment spot appears in the 

 nervous layer long before the closure of the anterior neuropore. 

 One of the causes operating to transfer the pigment from the 

 brain wall to the periphery is the constant growth from the 

 larva to the adult, and the consequent thickening and increas- 

 ing opacity of the superjacent tissue. 



After these diverticula have formed, there is left of the an- 

 terior brain wall between them a median portion which, as we 

 shall see, is the homologue of the lamina terminalis of the 

 remaining vertebrates. 



The lamina terminalis of this stage contains the basis of the 

 cerebral lobes, and the olf active* centres of all the higher forms. 



The relation of the parts h'ere described corresponds with the 

 facts as given in the latest and most accurate studies of verte- 

 brate ontogeny from all classes of vertebrates, which form a 

 valuable basis for the examination of this proposition from the 

 comparative standpoint. 



An examination of Hatschek' s figures, 64, 65, 67, and 69, for 

 an explanation of the relation of the parts, shows that the most 

 anterior portion of the neural plate of the young larva remains 

 for a long time in its primitive flattened, uninclosed condition. 

 Here the dorsal surface of the plate is still exposed to the direct 

 action of external stimuli ; its face is in or near the horizontal 

 plane, and hence looks vertically, or nearly vertically, upwards. 

 The eye-spot is developed in this area, and it clearly occupies 

 the most favorable position for an important sense organ that 

 the body of Amphioxus affords. 



As the anterior neuropore closes up more and more, the an- 

 terior lip grows upward curving backwards, the pigment spot 

 comes to lie in the vertical portion of the anterior end of the 

 now completed canal ; consequently it is strictly terminal in its 

 adult relations, but may, and frequently does, assume a position 

 at the side and behind the end of the axis. 



For greater functional power, the central (median) portion of 



the pigment spot has grown upwards (dorsad), and carrying with 



it a portion of the ventricular wall has produced the pineal eye. 



Let us follow through the development of the nerve cord of 



the larval Amphioxus. Formed as a thickened plate of cells on 



