58 



CHARLES BROOKOVER 



from what will be the anterior portion of the roof of the mouth, 

 and the neural tube, a small button of cells (fig. 1). No limiting 

 membrane was made out between this group of cells and the 

 brain wall. The cells were like those of what may be termed the 

 unpaired olfactory placode into which they grade. 



In a stage six hours older which has not been figured, the but- 

 ton appears as a cord of cells tapering downward and forward 



aired olfdc- 



ru placode 



Fig. 1. Transverse section (slanting slightly upward and backward) of the 

 anterior end of an embryo of Amia forty hours before hatching. Shows the paired 

 and unpaired nasal placodes, the anterior end of the neural tube, and the sucking 

 disk. Yolk granules are still evident in sucking disk. Iron hsematoxylin and acid 

 fuchsin, 6 micra thick. X 215. 



from beneath the anterior end of the neural tube to the unpaired 

 portion of the olfactory placode. A similar unpaired portion of 

 the olfactory placode has been described by von Kupffer ('93) in 

 the sturgeon. From this fact and the unpaired condition in 

 Amphioxus and the Cyclostomes he argues for primitive mono- 

 rhinism in the vertebrates and secondary amphirhinism. The 



