CEREBRAL VENTRICLES IN THE PIG 221 



hemisphere have been cut off, thus displaying the structures within 

 the ventricle. The corpus striatum is seen to be divided by a 

 vertical groove into two portions. The groove is deep below but 

 fades out above. Thompson ('09) has described a similar groove 

 in a cat embryo 20 mm. in length, and he refers to the larger 

 lateral and the smaller median subdivisions which are set apart 

 by the groove as 'roots' of the corpus striatum. His ('89) de- 

 scribed the corpus striatum as being composed of three limbs. In 

 the 45-mm. pig there are also three limbs, but only two are seen 

 in figure 14. The small median limb, which is not shown, fuses 

 with the lamina terminalis just below the interventricular fora- 

 men. The groove is shown at x in figure 18, which is a frontal sec- 

 tion through the head of a 45-mm. pig. The plane of this section 

 is indicated in figure 14. The groove is best seen in the right side 

 of the section, where the median subdivision may be observed 

 to pass down toward the paraterminal body. In a section fur- 

 ther back (fig. 19) the groove is quite shallow. Above the corpus 

 striatum (figs. 14 and 19) the chorioid plexus is seen projecting 

 into the lateral ventricle. The fold, surmounted by a frill, 

 which was seen in the 22-mm. embryo, has given place to a very 

 thin layer with many reduplications and subdivisions. Villi 

 are nowhere present, but in thin sections the smaller processes 

 extending out from the main folds may simulate them. Under 

 the binocular microscope it is clearly seen that this plexus consists 

 only of folds with secondary subdivisions. The plexus is attached 

 along a fissure measuring 2.1 mm. in length, which extends back- 

 ward from the interventricular foramen. 



The caudal portion of the plexus — about one-fourth of the 

 entire length — is very much attenuated. It forms a short free 

 projection extending 0.18 mm. beyond the end of the fissure, and 

 the free portion shows no secondary folds (fig. 21). The anterior 

 portion of the plexus forms a larger free projection which extends 

 0.72 mm. past the front end of the fissure. 



In the 22. mm. embryo (fig. 13) a slight invagination of the 

 medial wall of the hemisphere, above the chorioid plexus and 

 parallel with it, extends from the interventricular foramen back- 

 ward into the inferior horn. This represents the hippocampus, 



