SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 391 



men interventriculare. Some attention must be given separately 

 to each of the forms studied. 



In the turtle Cfigs. 9 and 17) the neuroporic recess is nearly 

 obliterated in the adult, apparently through the anterior and 

 pallial commissures approaching one another in later embryonic 

 stages. The dorsal commissure has the form of a loop whose 

 two limbs rise dorsally at either side to reach the hippocampus 

 (figs. 9 and 16). All recent authors agree that the compact layers 

 of cells occupying the medio-dorsal wall of the hemisphere repre- 

 sents some part of the hippocampal formation of mammals. 

 Meyer ('92), Elliot Smith ('96) and Levi ('04) rightly hold that 

 the lower portion of this cortex is the forerunner of the fascia 

 dentata. The ventral boundary of the hippocampal formation 

 is sharply marked both by the sudden change to a body con- 

 taining cells irregularly scattered through the thickness of the 

 wall and by a well-defined sulcus. The mass of scattered cells 

 is the primordium hippocampi above described. Its dorsal por- 

 tion in this section is filled by fibers of the fimbria system. This 

 is an old system of fibers connecting olfactory centers in front 

 with the whole length of the hippocampus. It is well developed 

 in the embryo before the hippocampal commissure is formed. 

 The constitution of this precommissural representative of the 

 fimbria is discussed below. The sulcus above mentioned, since 

 it lies between the fimbria and the developing fascia dentata, 

 must be regarded as the homologue of the fimbrio-dentate sulcus 

 of mammals. This sulcus has been variously treated by previous 

 authors. Elliot Smith COS) calls it the sulcus limitans in Sphe- 

 nodon, Edinger ('96) and linger ('06) call it the fissura arcuata, 

 Kappers and Theunissen ('08) call it fissura septo-corticalis, de 

 Lange ('11) uses the same name but calls it also fissura arcuata. 

 Herrick ('10) discusses the matter, and retains the term fissura 

 arcuata. The sulcus in question can not be the fissura arcuata, 

 since that is situated within the hippocampal formation, dorsal 

 to the fascia dentata. The fissura arcuata is not present in the 

 reptiles studied by the writer. On the other hand, the fimbrio- 

 dentate sulcus is a constant feature in the brains of reptiles and 

 mammals. It is the sulcus shown by Elliot Smith ('98, pi. XT, 



