THE ORGANS OF THE OUTER GERM-LAYER. 443 



below, is the temporal lobe (schl.l) ; the region lying above and 

 connecting the two is the parietal lobe (schei.l). A prominence 

 which is developed from the ring-lobe backward becomes the occipital 

 lobe (AJ). 



The lateral ventricle has also become altered and corresponds to the 

 external form of each hemisphere (fig. 253). .It also assumes the 

 shape of a half-ring, which lies above and surrounds the corpus 

 striatum (cst) that part of the wall of the vesicle which is forced 

 inward by the fissure of SYLVIUS. Subsequently, when the individual 

 lobes of the hemispheres are more sharply differentiated from one 

 another, the lateral ventricle also undergoes a subdivision correspond- 

 ing to the lobes. It becomes slightly enlarged at both ends, in front 

 into the anterior cornu occupying the frontal lobe, behind and below 

 into the inferior cornu of the temporal lobe. Finally, from the half- 

 ring there is developed a small evagination, the posterior cornu, 

 which extends backward into the occipital lobe. The region lying 

 between the horns is narrowed and becomes the cella media. 



All the fissures hitherto mentioned, except that of SYLVIUS, are 

 developed on the plane [median] surface of the vesicle of the 

 hemisphere. 



At a very early stage in Man in the fifth week (His) there arise 

 on this wall of the hemisphere two furrows running almost parallel 

 with the edge or bend of the mantle, the arcuate or hippocampal fissure 

 and the fissure of the choroid plexus (fissura hippocampi andfissura 

 choroidea) ; both conform very closely in their direction to the ring- 

 lobe, and, like it, with crescentic form embrace from above the stalk 

 part of the cerebrum, the corpus striatum. They begin at the 

 foramen of MONRO and extend from there to the tip of the temporal 

 lobe, forming the boundaries of a region known as the marginal arch 

 (Randbogen) ; this projects as a thickening on the median surface of 

 the hemisphere, and takes part in the development of the commissural 

 system. The invaginations of the median wall of the ventricle, caused 

 by the fissures, the hippocampal fold and the fold of the lateral choroid 

 plexus, are best understood by removing in an embryo the lateral 

 wall of the hemisphere, so that one can survey the inner surface of 

 the median wall of the still very spacious and ring-like lateral 

 ventricle (fig. 253). The cavity is then seen to be partly filled with 

 a reddish frilled fold (agf),which lies in the form of a crescent on the 

 upper surface of the corpus striatum (cst). In the region of the fold 

 the wall of the brain undergoes changes similar to those in the rocf 

 of the medulla oblongata and of the vesicle of the between-brain 



