180 bulletin: museum of comparative zoology. 



studied. This material was preserved for me in Zenker's fluid, and 

 has been stained for the most part with iron hematoxylin. 



In the constriction between the second and third primitive brain 

 vesicles (Plate 4, Fig. 23), which later become respectively the dieu- 

 cephalon and mesencephalon, there is just before hatching a differ- 

 entiation in the dorsal portion of the constricted nerve-tube. The 

 dorsal wall thickens rapidly, and the flexure becomes more accentuated, 

 growing downward and forward and further constricting the passage 

 between diacoele and mesocoele. In this thickening there arises during 

 the first day ^ of larval development a partial constriction, dividing it into 



Figure B. Amia calva, larva of first day. Sagittal section of the brain, showing 

 the position of the tectal reflex cells, and the great size of the encephalic cavi- 

 ties. For meaning of abbreviations, see Explanation of Plates, p. 257. 



two unequal portions (Fig. B). From the smaller anterior portion, 

 belonging to the diencephalic roof, is formed the epiphysis (Plate 11, 

 Fig. 71). The posterior thickened portion develops rapidly, finally 

 becoming sharply flexed as it grows ventrally and posteriorly. It is 

 through this downward flexure of the roof that the posterior commissure 

 develops about the thii'd day (Plate 4, Fig. 24, corn's. 2^-)' 



In this thickened fold are developed at an early stage numerous 

 ependymal cells, from which the ependymal groove is formed. Running 



1 The age here given is from the time of hatching, whicli according to Reighard 

 (:03) and Dean ('96) corresponds usually with the eighth to tenth day after 

 fertilization. 



