156 MARION HINES 
In this case the ventral boundary of the hippocampal formation 
may be drawn by passing a plane through the wall at the level 
of the sulcus limitans hippocampi. Such a limit would corre- 
spond to one similarly drawn diagonally through the medial wall 
of the brain of Phrynosoma cornutum (Herrick, ’10, fig. 61, p. 
533) joining the sulcus limitans hippocampi and a groove on the 
ventricular surface just dorsal to nucleus lateralis septi. In the 
brain of the turtle (Johnston, 13, p. 391, and fig. 17, p. 4385) 
a sulcus which lies above this hypothetical plane described by 
Herrick is called the sulcus fimbrio-dentatus. This is Herrick’s 
fissura arcuata. 
If, now, Johnston and others are correct in assuming that the 
mammalian fascia dentata is derived from the ventral part of 
the reptilian area of differentiated cortex above this so-called 
fimbrio-dentate sulcus, then the reptilian primordium hippocampi 
gives rise only to the mammalian fimbria and fornix bed and the 
term fimbrio-dentate sulcus is clearly appropriate, for this sulcus 
is defined by Johnston (713, p. 391) as ‘‘lying between the fimbria 
and the developing fascia dentata.’”” But, on the other hand, it 
has been shown in this contribution that the human fascia dentata 
actually is developed, not from the differentiated hippocampal 
cortex downward, but upward from the extreme ventral border 
of the primordium hippocampi. The hippocampal primordia 
of reptiles and of these human embryos are apparently strictly 
comparable structures. The fimbrio-dentate suleus as defined 
by Johnston cannot, therefore, lie dorsal to the primordium hip- 
pocampi as he describes it. 
We conclude, then, that there is no fimbrio-dentate sulcus either 
in reptiles or in the human embryos here described. The mode 
_ of its appearance in later developmental stages has not been 
determined in sufficient detail to enable the writer to treat the 
subject exhaustively, although sketch D in figure 51, taken from 
an embryo 85 mm. in length, clearly delineates the fact that the 
sulcus in question develops later than the fascia dentata and 
appears ventral to both the fascia dentata and the sulcus limitans 
hippocampi. It would appear to follow from the conclusion that 
if the reptilian fissura areuata of Herrick’s description (Ambrio- 
