THE ORGAKS OP THE OUTER GERM-LAYER. 



445 



the foramen of Monro to the tip of the temporal lobe and leads from 

 the outside into the lateral ventricle. This is the lateral cerebral fis- 

 sure, or the great Jissure of the hemispheres (fissura cerebri transversa). 



In a preparation made in the manner described the hippocampal 

 fold is to be seen at a short distance from the choroid plexus and 

 parallel to it (figs. 253 and 255 am/" and fig. 254 h). This increases 

 in size toward the apex of the inferior cornu, and in the completely 

 formed brain produces the cornu Ammonis or pes hippocampi. 

 Consequently that part of the lateral ventricle enclosed in the tem- 

 poral lobe becomes (as the result of two infoldings of its median 

 wall) restricted 

 by two pro- 

 jections, the 

 choroid plexus 

 and the cornu 

 Ammonis. As 

 in the between- 

 brain and me- 

 dulla o b 1 o n- 

 gata, the epi- 

 thelial covering 

 of the choroid 

 plexus is con- 

 tinuous, with 

 the thicker 

 nerve-sub- 

 stance of the 

 cornu A m- 

 monis. The 



transition is effected by means of a thin medullary plate, which in 

 anatomy is described as the fimbria. 



Inasmuch as the occipital lobe with its cavity develops as an 

 evagination of the ring-lobe, the fissura calcarina belonging to it 

 is therefore developed somewhat later than the arcuate fissure 

 (fig. 241 fc). It appears at the end of the third month as a fissure 

 branching off from the latter, and runs in a horizontal direction until 

 near the apex of the occipital lobe. It invaginates the median wall 

 of the lobe and produces the calcar avis, which invades the posterior 

 cornu in the same way as the hippocampus major (cornu Ammonis) 

 does the inferior cornu. At the beginning of the fourth month the 

 fissura occipitalis (fig. 241 fo) is added to it. The latter rises from 



Fig. 25S.— Transverse section thiough the brain of a Babbit embryo 



3-8 cm. in length, after Mihalkovics. Magnified 9 diameters. 

 The section passes through the foramina of Monro. 

 lis, Great falx cerebri which fills up the interpallial fissure ; *', h', plane 



inner [median] and convex outer wall of the cerebral hemisphere ; 



agf, fold of the choroid plexus ; amf, hippocampal fold ; /, fornix ; 



sVj lateral ventricle ; ML, foramen of Monro ; i;% third ventricle ; 



eh, optic chiasma ; fraf, descending root of the fornix. 



