CH. XV] ORGAXS FROM THE ECTODERM 161 



small round knob. This knob, the pineal body, lies just below 

 the surface-ectoderm. Later the structure grows forward, and 

 becomes dilated at its distal end. The dilated end or bulb re- 

 mains connected with the brain by a stalk. White particles 

 develop in the bulb, so that it stands out in strong contrast to 

 the dark surface of the brain. 



At the time of closure of the medullary folds, a mid-ventral 

 diverticulum forms from the floor of the fore-brain. This is 

 the infundibulum. It is in close contact with the anterior end 

 of the notochord (Fig. 38). The infundibulum is throughout 

 its subsequent history a w r ide sac with thin walls. It soon 

 comes into close connection with another structure, the pitui- 

 tary body (Fig. 39). The pituitary body arises very early, 

 even before the neural tube is closed, as a solid ingrowth, or 

 cord of cells, from the ectoderm, immediately in front of the 

 anterior end of the medullary plate (Fig. 37, A). Later, this 

 small, solid, tongue-like process projects inward from the 

 ectoderm beneath the brain and above the dorsal wall of the 

 pharynx. The inner end of the ingrowth expands into a flat- 

 tened mass of cells, which lies immediately beneath the anterior 

 end of the notochord. This mass becomes later the pituitary 

 body, while the rest of the process forms a slender stalk con- 

 nected at one end with the ectoderm. 



THE EYES 



The e}^es develop in part from the walls of the fore-brain. 

 Even before the neural tube is closed, in the embryos of some 

 species of frogs, two pigmented areas may be seen on the an- 

 tero-lateral walls at the anterior end of the infolding medullary 

 plate. These pigmented areas mark the region from which a 

 pair of evaginations of the fore-brain will develop to form the 

 optic vesicles. The hollow vesicles push out laterally toward 

 the sides of the head. Each tubular evagination then becomes 

 constricted, forming a distal hollow bulb and a proximal hollow 

 stalk (Fig. 49). The bulb gives rise to the retina and to the 

 pigment behind the retina, while, according to Marshall ('93), 

 the stalk forms a path along which the fibres of the optic 

 nerve pass from the eye to the brain. The outer hemisphere 

 of the optic bulb flattens and then pushes in so that the former 



