PIG EMBRYO, TEN-MILLIMETER 33 



is the olfactory pit. Caudal to the lachrymal groove and 

 dorsal to the olfactory pit is a thickened part, the maxil- 

 lary process which will form the greater part of the upper 

 jaw. Caudal to this thickening is a second one, the 

 mandibular process, which will form the lower jawi The 

 first gill cleft remains as a groove calidad to the mandibu- 

 lar process. This marks the position of the future ex- 

 ternal auditory meatus. The second, third and fourth 

 gill clefts have disappeared and their remains are marked 

 by a triangular depression, the apex of which is at the 

 ventral terminus of the first gill cleft. This depression 

 is the cervical sinus. 



Note that the length of the dorsal side of the embryo 

 is three or more times the length of the ventral side. On 

 the dorsal side from the hind-brain terminating in the 

 slender tail are a number of segments, the muscu lar 

 segments. On the ventral side of the embryo is the 

 large umbilical cord. Its connection with the embryo 

 occupies about one-third of the ventral part of the 

 body or almost all of the ventral surface of the abdominal 

 region proper. Anterior to the umbilical cord the cardiac 

 region protrudes ventrally. Between the cardiac region 

 and the muscular segments is a large evagination, the 

 fore-limb bud. Between the posterior connection of the 

 umbilical cord to the body and the origin of the tail is 

 another smaller evagination, the hind-limb bud. Extend- 

 ing from the fore-limb bud to the hind-limb bud along 

 curvature of the body is the milk ridge. 



Draw the whole embryo to scale and label all the parts. 



