THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



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 hyomandibular cleft which persists as the external auditory meatus. 

 About the dorsal end of the cleft develops the external ear. 



Cephalic flexure 



Maxillary process Mandibular process 



Branchial arch 2 

 Branchial arch 3 



Eye 



Olfactory pit 

 Yolk sac 



4^ U7. 



V ' \u 



Cut edge of amnion 

 Lower limb bud 



jCervical sinus 



, Atrium of heart 



Liver 



"pper limb bud 

 Mesodermal segment 

 Mesonephros 



FIG. 93. Pig embryo of 6 mm., viewed from the left side. The amnion has been removed. 



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 liver is the 

 bud of the upper limb, 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 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 some- 

 what precociously developed in comparison with a human embryo of the 

 same size (Fig. 94). In a human embryo 7 mm. long the head is larger, 



