Johnston, Forehrain Vesicle in Vertebrates. 



491 



along Avhich ectoderm and neural plate meet and, when the neural 

 plate rolls up into a tube, the ridge forms the lower border of the 

 neuropore. These relations are as simple and clear in Amblystoma 

 as in Squalus (Figs. 23, 24). Even after the neuropore has closed 

 the arrangement of cells and nuclei in this region shows the outline 

 of the terminal ridge. After the brain is separated from the ecto- 

 derm the terminal ridge forms a distinct fold, convex toward the 

 ventricle (Fig. 26), which in later stages is occupied by the fibers 

 of the optic tracts in the chiasma (Fig. 33). ]^o neuroporic recess 

 is to be seen in A. punctatum in early stages following the closure 



Fig. 26. Fig. 27- 



Fig. 2G. Amblystoma p., invagiuatiou of hypophysis beginning; primitive 



inferior lobes. Sagittal section of head, x 25. 

 Fig. 27. Amblystoma p., hypophysial invagination at its height; velum 



transversum and epiphysis ; median sagittal section reconstructed from several 



sections. X 25. 



of the neuropore, but in later stages a slight pit is found which 

 may correspond to the neuroporic recess described in other forms 

 (Fig. 33). 



The early appearance of the retinal areas on the neural plate 

 was first described by Eycleshymer (1890) and the fact that the 

 optic vesicles are formed from the lateral parts of the neural plate 

 has been pointed out by the writer (1905, 1906). Wliile the neural 

 plate is still open the retinal pits are connected with one another 

 by a shallow groove running just behind the terminal ridge (Fig. 

 23, r. po). As the neural plate rolls up and the optic vesicles are 

 evaginated this groove grows deeper (Figs. 24, 25) and by the time 



