THE FISSURA HIPPOCAMPI 97 
The changes which characterize the two embryos last described 
are: 
1. Closure of the choroid fissure. 
2. Actual evagination of the olfactory bulb. 
3. Plexus formation in the anterior portion of the diencephalic 
roof plate. 
4, Great increase in growth of the midthalamie region. 
5. Reversal of relative size of the two limbs of the caudate 
nucleus. 
6: Further thickening of the dorsal portion of the lamina 
terminalis, i.e., the encroachment of the pars crassa upon the 
pars tenuis. 
7. The incipience of the temporal lobe in the cerebral hemis- 
phere. 
8. The fissura hippocampi is not so deep above the area cho- 
rloidea in the 27.8 mm. embryo as in the 19.1 mm. or the 20 mm. 
Anterior to the angulus terminalis and posterior to the velum 
transversum the fissura resembles that found in the embryos 
previously described. The formation, however, is continuous 
throughout. In the 32.1-mm. the fissura hippocampi is very 
shallow anterior to the velum transversum, posteriorly it is 
relatively a deep groove. 
The 39.1-mm. embryo, University of Chicago, H 163 (fig. 18) 
The changes in the medulla oblongata and the midbrain may 
be seen at a glance. The cerebellum (Cer.) appears as a medially 
growing thickening of the dorsal lip of the lateral recess, but as 
yet no fusion has taken place in the midline. The floor plate 
in both the medulla oblongata (Myel.) and the midbrain (Mes.) 
has thickened. The floor plate of the latter does not show 
any of the subdivisions characteristic of the adult mesencephalon. 
The sulcus limitans (Sul. lim.) can be easily followed from the 
cord through the myelencephalon and mesencephalon into the 
posterior part of the diencephalon. 
The greatest change as compared with H 41 (32.1 mm.) is 
found in the development of the prosencephalon, especially in 
the telencephalic portion. In the diencephalon the sulcus divid- 
