456 C. JUDSON HERRICK 



scheme of subdivision of the reptilian prosecephalon seems to me 

 faulty, as will appear beyond. This is probably in part due to 

 the fact that they did not study sufficiently early stages but chiefly 

 because of imperfect knowledge of the functional factors involved 

 in development such as would be brought out only by a study of 

 the adult fiber tracts. 



Lacertilia. — In the lizards the precommissural body is much 

 more highly developed than in the turtles and this leads to im- 

 portant differences in the development of the septal region. 

 In the adult the nuclei of the septum extend much farther dorsad 

 and caudad than in turtles in relations similar to those of the pars 

 fimbrialis septi of the frog. 



The Harvard series 1601 comprises frontal sections of an em- 

 bryo of Lacerta of 7.6 mm. (measured as coiled, total length un- 

 known) cut in approximately the same plane as the older embryo 

 shown in figs 53 to 57, though somewhat more nearly transverse 

 to the long axis of the diencephalon. At this age the commissura 

 pallii anterior is present, though small, but the commissura pallii 

 posterior has not yet appeared. Tandler and Cantor ('07) find 

 that in the gecko also the commissura pallii anterior develops ear- 

 lier than the commissura pallii posterior. 



A section through the mid-region of the septum, corresponding 

 approximately to the plane of fig. 44 of Chrysemys, shows that 

 the nucleus lateralis septi has grown dorsad so as to invade the 

 ependymal border of the primordium hippocampi (fig. 42), a 

 condition which is found in this region of adult Chrysemys but 

 not in the embryo of the age shown in fig. 44. This position of the 

 lateral nucleus is characteristic of the entire length of the septum of 

 Lacerta at this and all later ages. The ventral limit of the original 

 primordium hippocampi can, however, generally be recognized 

 on the outer surface of the hemisphere of adult lizards. The re- 

 gion corresponding with the membranous septum ependymale of 

 of the Chrysemys embryo of 16.7 mm. (fig. 45) is massive in the 

 Lacerta embryo of 7.6 mm., containing fimbria fibers and cells 

 of the medial and lateral nuclei of the septum. 



Passing caudad in this series of sections of Lacerta, at the level 

 of the commissura pallii anterior a portion of the nucleus median us 



