336 Robert J. Terry. 



Superior Commissure. This tract is not present. 



Posterior Commissure. In an embryo of 6.5 mm. the posterior 

 commissure appears in sagittal sections (fig. 13, PC) as a large 

 clear area in the ectoglia of the brain-roof, limited within the 

 deep fold between the fore- and mid-brain. Throughout this 

 area a network of fine processes, continuous with the ependyma, 

 can be seen. The dorsal surface of the commissure presents an 

 indentation, the beginning of the division into the two parts seen 

 in later stages. This indentation is found in Amia (fig. 11, P.C) 

 'and in the trout as represented in Kapffer's ('06) fig. 150. This 

 investigator says, (p. 131): ''Sowohl die Commissura anterior 

 wie die posterior lassen bei der Forelle anfanglich zwei scharf 

 geschiedene Portionen erkennen." In Opsanus the anterior 

 part reaches the epiphysis and lies in contact with its posterior 

 surface. 



In embryos of 5 mm. the commissure is stretched out in the super- 

 ficial or ectoglia layer of the brain-roof, caudad of the epiphyseal 

 arch (fig. 12, P C). Haller ('98) has seen a similar disposition in 

 selachians and remarks that it is transitory in the teleosts. The 

 extended posterior commissure has been observed in other forms, 

 as, for example, Ammocoetes (See Kupffer, '06, fig. 47), and it 

 has been found in the present investigation in Fundulus embryos 

 of 7 mm. The fore-brain in front of the epiphysis presents in 

 sagittal sections an angular bend dividing it into a longer posterior 

 and a shorter anterior segment (fig. 12). Behind the bend a 

 large blood sinus lies in contact with the roof which at this point 

 projects into the ventricle. This is the beginning of the velum 

 transversum. The epithelial roof in front of the angle goes over 

 anteriorly by a slight curve into the thicker terminal wall of the 

 ventricle. This curve is to be compared with Minot's paraphyseal 

 arch. 



Opsanus Embryos S.5 mm. in Length 



Sagittal sections show the brain roof considerably thickened 

 in the epiphyseal region where a slight folding represents the 

 beginning of the epiphyseal arch (fig. 14, EA). A short distance 



