9 8 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



mandibular arch, forks ventrally into two processes, a smaller maxillary and a 

 larger mandibular process, and the latter with its fellow forms the mandible or 

 lower jaw. The position of the mouth is indicated by the cleft between these 

 processes. The groove between the eye and the mouth is the lacrymal groove. 



The second or Jiyoid arch is separated from the mandibular arch by a hyo- 

 mandibular cleft which persists as the external auditory meatus. About the dorsal 

 end of the cleft develops the external ear. 



The heart is large and through the transparent body wall may be seen the 

 dorsal atrium and ventral ventricle. Caudal to the heart, a convexity indicates 



Cephalic flexure 



Eye 



Olfactory pit 

 Yolk-Sac 



Miliary process ^^ 



'or process 

 r.arch II 



r. arch 111 



Cervical sinus 



Atrium of heart 



Mesonephros 



Fig. SS. — Pig embryo of 6 mm., viewed from the left side. The amnion has been removed and its cut 



edge is shown in the figure. X 12. 



the position of the liver. Dorsal to the liver is the bud of the anterior extremity, 

 now larger than in the 4 mm. human embryo. Extending caudal to the anlage 

 of the upper extremity, a curved convexity indicates the position of the right 

 mesonephros. At its caudal end is the bud of the lower limb. The amnion has 

 been dissected away along the line of its attachment ventral to the mesonephros. 

 There is as yet no distinct umbilical cord and a portion of the body-stalk is at- 

 tached to the embryo. 



As the term of its development is shorter, a young pig embryo is somewhat 

 precocious in its development as compared with a human embryo of the same 

 size (Fig. 89). In a human embryo 7 mm. long the head is larger, the tail shorter. 



