THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS 



Central Nervous System. Dissections show well the form and rela- 

 tions of the organs (Figs. 120, 121 and 122). Directions for preparing dis- 

 sections are given in Chapter VI. 



The Brain. Five distinct regions may be distinguished (Figs. 120 

 and 122): (i) 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 dien- 

 cephalon shows a laterally flattened cavity, the third ventricle. Ventro- 

 laterally from the diencephalon pass off the optic stalks, and an evagina- 

 tion of the mid-ventral wall is the anlage of the posterior hypophyseal 



Accessory gang, i 

 Accessory gang. 2 



Ace. gang. 3 



Myelencephalon 



Ace. gang 4 



Gang, nodosnm 



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. 



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-laterally it becomes the cerebellum, ventrally the pons. 

 (5) The elongated myelencephalon 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. 



The Cerebral Nerves. Of the twelve cerebral nerves, all but the first 

 (olfactory) and sixth (abducens) are represented in Fig. 120. For a de- 

 tailed description of these nerves see Chapter XIII. (2) The optic 



