THE FISSURA HIPPOCAMPI 101 
5. No suleus dorsalis thalami can be identified upon the ven- 
tricular surface. 
6. Appearance of the island of Reil. 
7. Great growth of the neopallium. 
The 43-mm. embryo, Mall Collection, 886 (figs. 19 and 20) 
This embryo measured 39.9 mm. greatest length in alcohol, 
only 0.8 mm. longer than H 163. It is not strange that the fore- 
brains of these two embryos are very similar. The development 
of the cord and the medulla oblongata is the same. ‘The cere- 
bellum appears as a medial outgrowth from the dorsal lip of the 
lateral recess. The floor plate is very thick. The midbrain 
is divided into alar and basal plates by the sulcus limitans. The 
basal plate is increasing in depth. There is no hint of a division 
into colliculi upon the roof of the mesencephalon. ‘The ventric- 
ular markings in the thalamus are the same. The sulcus limi- 
tans ends blindly in the hypothalamus. It is joined for a short 
_ distance by the sulcus which delimits the ventral boundary 
of the midthalamus, the sulcus Monroi. Dorsal to this sulcus is 
the great midthalamic mass. Here as in the 39.1 mm. embryo 
there is no visible separation between this part of the thalamus and 
the epithalamus. The epithalamus contains the characteristic 
structures, the epiphyseal evagination, the habenula, the habe- 
nular commissure, and the choroid plexus of the third ventricle. 
The hypothalamus contains the corpus mamillare, the thin 
tuber cinereum, the posterior lobe of the hypophysis, the re- 
cessus infundibuli. The last recess is deeper in this embryo than 
in H 163 (39.1 mm.). The recess preopticus lying between the 
chiasma ridge and the inferior limb of the lamina terminalis is 
longer and more shallow than that of the 39.1-mm. embryo. 
The structures of the telencephalon medium are similar in 
relation and development to those found in the 39.1-mm. embryo. 
The lamina terminalis is slightly longer, but not broader. The 
angulus terminalis is not as well delineated. The length of the 
area chorioidea is practically the same as that of H 163. Its 
anterior division, the tela chorioidea telencephali medii is like the 
39.1-mm. but its posterior, the paraphyseal arch, is not the same. 
