9° 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



angular depression, the cervical sinus, formed by the more rapid growth of the first 

 and second arches (cf. Fig. 97). The first, or 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 lacrimal groove. 



The second or hyoid 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. 



Cephalic Jlcxiirr 



Olfactory pil 

 Yolk sa, 



Maxillary process Mandibular process 



Firanrlnal arch 



Cut edge of amn'uni-j— 

 Lower limb hud 



Branchial arch 3 

 Cervical iiiiiis 



Iriiim of heart 

 ^ Liver 



:x^f' pper limb hud 



Mesodermal segment 



yy^~~~- M csonephros 



Fig. 93. — 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 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 

 the position of the liver. Dorsal to the Kver 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 tlae left 

 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 

 attached to the embryo. 



Due to a shorter term of development, a young pig embryo is somewhat pre- 



