122 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



the head, the shorter tail, the much smaller mesonephric region, the longer 

 umbilical cord and the less prominent segments. The yolk-sac is pear-shaped 

 with long slender yolk-stalk. 



Central Nervous System and Viscera. — Dissections show well the form and 

 relations of the organs (Figs. 115, 116 and 117). Directions for preparing dis- 

 sections are given in Chapter VI. 



Metencephalon N. trochlearis 



Gang. n. 5 \ I Mesencephalon 



Gang. 

 Gang, jugula 

 Gang, petrosa 



Gang. Fror 

 Gang, nodos. n. / 



olonus 



icnccphalon 



Ophthalmic r. n. 5 

 opticus 



axillary r. n. 5 

 Icnccphalon 



andibnlar r. n. 5 

 orda lymp. n. 7 



nlricle 



Gang, thora 



mbilical cord 



eminence 



Fig. 115. — Lateral dissection of a 10 mm. pig embryo, showing the viscera and nervous system from 

 the right side. The eye has been removed and the otic vesicle is represented by a broken line. The 

 ventral roots of the spinal nerves arc not indicated. X 10.5. 11., nerve; r., ramus. 



Brain. — Five distinct regions may be distinguished (Figs. 115 and 117): 

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

 Their cavities, the lateral ventricles communicate by the interventricular foramen 

 with the third ventricle. (2) The dienccphalon 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 



