THE FISSURA HIPPOCAMPI nar 
dentate suleus of Johnston) is represented in the human embryo 
at all, it must be homologous with the fissura arcuata of His and 
with the fissura hippocampi of the adult. If, however, this 
conclusion is adopted, it must be recognized that the fissure 
is very differently disposed with reference to the chief mass of 
the differentiated hippocampal cortex in adult reptiles and mam- 
mals. But the embryology of the region as far as followed in 
this paper is almost the exact duplication of the situation as 
described for reptiles by Herrick (10, pp. 464, 465). And the 
writer is inclined to think that future study will prove those 
differences discussed by Herrick to be slight indeed, for the 
fascia dentata arises from cells in the ventral lip of the fissura, 
immediately opposite or slightly dorsal to the ventricular sulcus 
limitans hippocampi. The fibers of the fornix lie between the 
matrix and the fascia dentata in this region. Moreover, the 
ventral lip of the fissura hippocampi shows no cortical lamination 
in the stages presented. ‘This differentiation of the early hippo- 
campus into a dorsal cortical portion and a ventral non-cortical 
resembles the reptilian condition, with the exception that opposite 
the sulcus limitans hippocampi lies the undifferentiated fascia 
dentata. There is nothing, according to Johnston or Crosby 
which compares to this differentiation of fascia dentata in either 
the turtle or the alligator. The writer suggests that the regions 
in adult reptiles called by these authors primordium hippocampi 
are the source of the fascia dentata. 
There are some points, essential for the completion of this 
argument which remain obscure. In the material avialable it 
is impossible to determine the absolute ventral limit of the 
primordium hippocampi caudal to the angulus ferminalis. 
The paraphysis is universally regarded as a differentiation 
within the roof plate. The lamina epithelialis in all probability 
should not be so regarded since it takes part in the evagination 
of the hemisphere. Its subsequent position, however, is not 
evident until the 14-mm. embryo is studied. The sulcus limitans 
hippocampi in this contribution is regarded as markng the 
ventral boundary of the hippocampal formation. In the region 
rostral to the angulus terminalis (figs. 3 to 6, sketches 1 v) it 
