222 CHESTER H. HEUSER 



which is better developed in the 45-mm. embryo. It is seen in 

 figures 19, 20, and 21 (marked +). 



Considered as a whole, the lateral ventricle has expanded 

 greatly and may readily be divided into three parts — the anterior 

 horn, which is in front of the corpus striatum; the body, which is 

 above it; and the inferior horn, descending behind it. A cast of 

 the left lateral ventricle is shown in side view in figure 16, and in 

 ventral view in figure 17. The latter shows the large excavation 

 made by the corpus striatum. Toward the olfactory lobe this 

 concavity is bisected by a ridge, which lies between the large roots 

 of the corpus striatum, as described in connection with figure 

 14. Below the concavity, in figure 17, there is a ridge which is 

 fissured, posterior to the interventricular foramen, to receive the 

 chorioid plexus. This ridge separates the hollow for the corpus 

 striatum above, from that for the hippocampus below. 



The lateral ventricle communicates with the third ventricle 

 by an interventricular foramen which is smaller than in the pre- 

 ceding stages. The third ventricle has become reduced to a slit- 

 like space, owing to the great thickening of the walls of the dien- 

 cephalon. A portion of it has become practically obliterated. 

 This is where the thalami have grown against each other (figure 

 21). They have not yet fused, however, to form the massa inter- 

 media; that is the ependymal layer over each thalamus is still 

 uninterrupted. Along the dorsal margin of the cleft, which rep- 

 resents the ventricle, there is a thin lateral expansion on either 

 side (fig. 15). The expansion begins at the interventricular fora- 

 men, where it is broadest, and it diminishes backward, ending 

 a short distance in front of the pineal recess. Thus the expansion 

 is wedge-shaped when seen from above. The brain-wall which 

 overlies this expansion is the tela chorioidea, which has a corru- 

 gated surface in relation with the vascular mesench^ma. Below 

 and behind the thalamus is a deep groove — the sulcus limitans — ■ 

 which, as already described, sends prolongations to the interven- 

 tricular foramen, recesses postopticus and recessus opticus. The 

 sulcus is seen in section in figure 20, with the thalamus above and 

 the pars optica hypothalami below. 



