44 Vertebrate Embryology 



THE NOSE 



The nose, like the eye and ear, begins at a 

 very early period, and is first seen as two 

 thickenings of the inner layer of the ectoblast, 



FIG. 16. THE BRAIN OF THE FROG. (After Marshall.) Figure 

 on left is a dorsal view ; figure on right is a ventral view. 



C, cerebellum. CH, cerebral hemisphere. CP, choroid plexus of third ven- 

 tricle. F, fourth ventricle. IN, tuber cinereum. M, medulla oblongata. O, 

 olfactory lobe. O C, optic chiasma. O L, optic lobe. P, stalk of pineal body. 

 PR, pituitary body. T, thalamencephalon. 



/, olfactory nerve. //, optic nerve. ///, third or motor oculi nerve. IV, 

 fourth nerve. V, fifth or trigeminal nerve. 1/1, sixth nerve. VII and VIII, 

 combined root of facial and auditory nerves. IX and X, combined root of glosso- 

 pharyngeal and pneumogastric nerves. 



one on each side of the anterior end of the 

 head. Aninvaginationof both layers of the ecto- 

 blast now takes place (Fig. 22, OF), forming 

 the nasal pits whose openings to the exterior 



