CEREBRAL VENTRICLES IN THE PIG 219 



tion. Abo^-e this hollow is seen the inferior horn of the lateral 

 \'entricle. The outline of the ventricle is no longer round, as in 

 the 12-mm. embryo. It is prolonged anteriorly or downward to 

 form the first indication of the olfactory lobe, and posteriorly or 

 above, it is somewhat flattened. This upper portion terminates 

 in the inferior or descending horn. Fraser ('94) has referred to 

 this horn in an abnormal human adult brain as in ' ' reality ascend- 

 ing" and Thompson ('08) similarly states that "from the develop- 

 mental point of view, this descending horn is in reality an ascend- 

 ing horn." In pig embryos, however, as seen in figures 5 and 9, 

 its primary direction is apparently ventral to the cerebral axis 

 so that the inferior horn may be said to descend, even in early 

 stages. The cavity of the lateral ventricle is seen laid open in 

 figure 10. It differs from that of the 12-mm. pig embryo (fig. 6) 

 since it has been invaded by a prominent chorioid fold. This fold 

 is a lateral extension of the velum transversum, and it consists of 

 vascular mesenchyma covered by the thin wall of the brain. At 

 this stage its ventricular surface is perfectly smooth, but in a 22- 

 mm. embryo the chorioid fold has developed a vascular fringe 

 which appears as a plaited frill. This is shown in figure 13. 



The walls of the diencephalon in the 17-mm. embryo have 

 thickened considerably so that the external depression between 

 the parencephalon and the synencephalon has become almost 

 imperceptible. Their cavities however, can be distinctly seen 

 when the dissection of the brain is studied (figs. 8 and 9). The 

 mamillary recess now projects out some distance from the third 

 ventricle. Midway between this recess and the infundibulum 

 the floor of the brain has thickened to form the tuber cinereum. 

 The optic thalami, represented by the thickenings of the lateral 

 walls of the diencephalon, have developed to such an extent that 

 they have already partially fused with the corpus striatum. The 

 place of fusion appears as an interruption of the groove between 

 the interventricular foramen and the optic recess. 



The median section of a brain of a 22-mm. embryo (fig. 11), 

 as compared with that of the 17-mm. specimen, shows several 

 new features. Along the cut edge dorsally, the pineal body 

 has appeared as a slight elevation. Within the third ventricle, 



