PIG EMBRYO, TEN-MILLIMETER 35 



vein on either side of the body ventral to the spinal cord. 

 Near the jugular vein is the united vagus and spinal 

 accessory nerve. Ventral to this nerve and also near the 

 jugular \'ein is the petrosal ganglion of the ninth nerve. 

 Identify the gill clefts and gill arches, the carotid artery, 

 the pharynx, the optic stalk, the two layers of the retina, 

 the choroid fissure, the lens, the cerebral hemispheres. 

 Ventral to the first gill cleft is the maxillary nerve. 



Draw the section, color the germ layers, and label all the 

 parts. 



(3) Through the Middle of the Heart and the Fore-limb Buds 



The description of the spinal cord as given in section 

 (2) will apply here also. At the origin of the limb bud 

 is a nerve plexus, the brachial plexus. Dorsal to the 

 brachial plexus is a blood vessel, the subclavian vein. 

 Ventral to the notochord and on either side of the median 

 line of the body is the dorsal aorta. Between the two 

 dorsal aortae and a little ventral to them is the oesophagus. 

 Ventral to the oesophagus is the trachea. Ventral to the 

 limb buds and on either side of the trachea is a large 

 irregular blood vessel, the duct of Cuvier. The heart is 

 now divided into four chambers, the two auricles and the 

 two ventricles. The walls of the ventricles have become 

 thickened and now have some of the characteristics of 

 heart tissue. The tissue between the two ventricles is 

 the ventricular septum. The triangular pieces of tissue 

 extending from the dorsal wall of each ventricle dowTi 

 into the cavity of the ventricle are the auricular-ventricu- 

 lar valves. The ca\aties of the auricles are larger than 



