Johnston, Forehrain Vesicle in Vertebrates. 



501 



15 to 17 mm. embryo the chorioid plexus of the lateral ventricle is 

 forming. Its position with reference to the velum transversum is 

 shown in a parasagittal section of a 17 mm. embryo (Fig. 40) and 

 in three frontal sections of the brain of a 15 mm. embryo (Fig. 41). 

 The velum transversum is not only a dorsal fold, but is continued 

 around the lateral wall of the brain as the constriction (external fur- 



FiG. 39. Pig embryo, 17 mm. Median sagittal section reconstructed from 

 several sections. The outline of the hemisphere lateral to the median plane 

 is shown in dotted lines, x 12%. 



row and internal ridge) between the diencephalon and the telen- 

 cephalon. By the stage mentioned the cerebral vesicle is sufficiently 

 expanded to push back past the boundary line. In the angle between 

 the vesicle and the diencephalon appears. the chorioid plexus pushing 

 into the lateral ventricle. It appears as a folding of the anterior 

 wall or limb of the velum transversum and its lateral prolongation. 



