146 



THE DISSECTION OF PIG EMBRYOS 



heart, noting its external structure. The liver may also be removed, leaving the stomach 

 and intestine uninjured. A portion of the septum transversum covering the lungs 

 may be carefully stripped away and the lungs thus laid bare. 



Dissections made in this way show the trachea and lungs, the esophagus, stomach and 

 dorsal attachment of the septum transversum, the course of the intestinal canal, and also 

 the mesonephroi and their ducts. Favorable sections through the caudal end of the body 

 may show the urogenital sinus, rectum, and sections of the umbilical arteries and allantois 

 (Figs. 97, 124 and 148). In late stages, by removing the digestive organs, the uro- 

 genital ducts and glands are beautifully demonstrated (Figs. 223 and 224). 



DEVELOPMENT OF THE FACE 



The heads of pig embryos have long been used for the study of the development of 

 the face. The heads should be removed by passing the razor blade between the heart and 

 the adjacent surface of the head, thus severing the neck. Next cut away the dorsal part 



Lateral nasal process 



Olfactory pit 



Medial nasal process 



Mandible 



Branchial arch 2 

 Ventral aorta- 



Eye 



Lacrimal' groove 

 Maxillary process 



Branchial groove i 

 Branchial groove 2 



Lateral nasal process 



Maxillary process 

 Mandible 

 Branchial groove I -\ 



External naris 



Eye 



Medial nasal process 

 Oral cavity 



External ear 



FIG. 149. Two stages showing the development of the face in pig embryos. 



B, 14 mm. 



X 7. A, 12 mm.; 



of the head by a section parallel to the ventral surface, the razor blade passing dorsal to 

 the branchial clefts and eyes. Mount, ventral side up, three stages from embryos 6, 12, 

 and 14 mm. long, as shown in Figs. 97 and 149. 



In the early stages (Figs. 97 and 124), the four, paired branchial 

 arches and grooves are seen. Each first arch has already bifurcated into a 

 maxillary and mandibular process. The third and fourth arches soon sink 

 into the cervical sinus, while the mandibular processes of the first arch are 



