VI.] THE PITDITAET BODY. 121 



which has abeady commenced to become vascular. The cords 

 or masses of epiblast cells are surrounded by a delicate mem- 

 brana propria, and a few of them possess a small lumen. The 

 infundibulum has increased in length. The relative positions of 

 the pituitary body and infundibulum are shewn in the figure of 

 the brain in Chapter viii. 



On the twelfth day the communication between the pituitary 

 vesicle and the throat is entirely obliterated, but a solid cord of 

 cells still connects the two. The vessels of the pia mater of the 

 vesicle of the third ventricle have become connected with the 

 pituitary body, and the infundibulum has grown down along its 

 posterior border. 



In the later stages aU connection is lost between the pituitary 

 body and the throat, and the former becomes attached to the 

 elongated processus infundihvli. 



The real nature of the pituitary body is stUl extremely obscure, 

 but it is not improbably the remnant of a glandular structure 

 which may have opened into the mouth in primitive vertebrate 

 forms, but which has ceased to have a function in existing 

 vertebrates'. 



Beyond an increase in size, which it shares vdth 

 nearly all parts of the embryo, and the change of 

 position to which we have already referred, the mid- 

 brain undergoes no great alteration during the third 

 day. Its roof will ultimately become developed into 

 the corpora bigemina or optic lobes, its floor will form 

 the crura cerebri, and its cavity will be reduced to the 

 narrow canal known as the iter a tertio ad quartimi 

 ventriculum. 



In the hind-brain, or third cerebral vesicle, that 

 part which lies nearest to the mid-brain, is during 



1 Wilhelin MiUler Veber die Entwicklwng und Bau der Hypophysis 

 v/nd des Processus Infundibuli Cerebri. Jenaische Zeitschrift, Bd. vi. 

 1871, and V. von Mihalkoyios, Wirbelsaite u. Himanhang, Archiv f. 

 mikr. Anat. Vol. xi. 1875. 



