Abnormal Development of Toad Ova 25 



internal structures. Posterior to the mouth the sucker is well 

 developed on each side (section b). 



Central Nervous System — The brain ends anteriorly in a hollow 

 rounded protuberance from which a vesicular ventricle projects 

 on each side (section a). The brain does not extend anteriorly to 

 the olfactory pits. The walls of the telencephalon and the hemi- 

 spheres are irregular in thickness and the cells and fibers are very 

 abnormally disposed. Many pigmented and appparently degener- 

 ate cells lie in the walls of the third ventricle and free in the ventri- 

 cle. The pineal gland lies between the roof of the third ventricle 

 and the ectoderm in a region posterior to section a. It has a thin 

 wall and a hollow central cavity containing scattered cells which 

 show evidences of degeneration. As one passes back the neural 

 canal becomes much dilated. In the region of the optic stalk it 

 is thin-walled (section b). The hypophysis is somewhat dilated 

 but is relatively normal in structure (section c). The roof of the 

 midbrain is rather thin. In the region of the ears the left side of 

 the brain is much less developed than the right side (section d) 

 and posterior to the ears for a considerable distance the left side 

 of the neural tube consists of hardly more than an irregular mem- 

 branous wall. Near the middle of the trunk the spinal cord is 

 more symmetrical, but posteriorly it is once more undeveloped on 

 the left side (section g). 



Organs of Special Sense — The nasal pits on each side are con- 

 nected with the pharynx by columns of cells (section a) which 

 show near the pharynx an imperfect lumen. The eyes are irregu- 

 lar in form (section b). A cornea has been differentiated. The 

 sensory portion of the retina is poorly developed. The lumen 

 of the optic stalk is dilated and extends into a space between the 

 two layers of the retina. There are no nerve fibers (section b). 

 The pigment cells of the pigment layer are irregularly disposed 

 and project out into the neighboring tissues. The auditory sac- 

 cule consists of a rounded pouch with a dorsal diverticulum (see 

 d). The auditory ganglion is clearly marked but no nerve fibers 

 can be distinguished. 



Peripheral Nerves — The cells of the ganglion of the trigeminal 

 nerve are large and well differentiated. Nerve fibers cannot 



