498 'Journal of Comparative Neurology and Psychology. 



The time of development of the mamm.illary recesses and of the 

 neural part of the hypophysis (saccus vasculosus) varies somewhat 

 in pig embryos. In most 6 mm. embryos the mammillary recess is 

 already clearly recognizable as a caudal expansion of the primitive 

 inferior lobe (Fig. 35, A) whose border above and behind is the 

 tviberculum posterius. From now on the mammillary recess is always 

 clear. Between it and the primitive optic groove the hypophysis 

 lies against the under surface of the primitive inferior lobe. In 

 some 6 mm. embryos there is to be seen just behind the tip of the 



Fig. 3G. Pig embryo, 9 umi., median sagittal section. X 20. 



hypophysis in sagittal section a slight, but definite, depression and 

 thickening in the brain floor, the beginning of the evagination of the 

 neural part of the hypophysis. This shows a variable development 

 in embryos between 6 mm. and 9 mm., but is always clearly present in 

 9 mm. embryos (Fig. 36). After this time the sac grows out and 

 enwraps the tip of the hypophysis and the relations familiar to all 

 embryologists are established. The outgrowth of this sac is the ear- 

 liest mark of the position of the infundibulum and this is a consid- 

 erable distance behind the primitive optic groove, as in selachians 

 and amphibians. In later development the floor of the primitive 



