THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS 



IIS 



Brain. — Five distinct regions may be distinguished (Figs. 120 and 122): 

 (1) The telencephalon with its rounded lateral outgrowths, the cerebral hemispheres. 

 Their cavities, the lateral ventricles, communicate by the interventricular foramina 

 with the third ventricle. (2) The diencephalon shows a laterally flattened cavity, 

 the third ventricle. Ventro-laterally from the diencephalon pass off the optic 

 stalks and an evagination of the mid-ventral wall is the anlage of the posterior 

 hypophyseal lobe. (3) The mesencephalon is undivided, but its cavity becomes the 

 cerebral aqueduct leading caudally into the fourth ventricle. (4) The metencephalon 

 is separated from the mesencephalon- by a constriction, the isthmus. Dorso-later- 



Accessory gang, i 



Ace. gang. 5 



Myelencephalon 



Ace. gang. 4 



Gang. Froriep 



N. II 



Cerv. gang. I 



Cerv. gang. 2 



Gang, nodosum 

 N. 12 



^ Fig. 121. — Dissection of the head of a 15 mm. pig embryo from the right side to show the accessory- 



vagus ganglia with peripheral roots passing to the hypoglossal nerve. X 25. 



ally it becomes the cerebellum, ventrally the pons. (5) The elongated myelen- 

 cephalon is roofed over by a thin non-nervous ependymal layer. Its ventro-lateral 

 wall is thickened and still gives internal indication of the neuromeres. The cavity 

 of the metencephalon and myelencephalon is the fourth ventricle. 



Cerebral Nerves. — Of the twelve cerebral nerves all but the first (olfactory) 

 and sixth (abducens) are represented in Fig. 120. For a detailed description 

 of these nerves see Chapter XIII. (2) The optic nerve is represented by the optic 

 stalk cut through in Fig. 120. (3) The oculomotor, a motor nerve to four of the eye 

 muscles, takes origin from the ventro-lateral wall of the mesencephalon. (4) 



