DEVELOPMENT OF THE FACE 1 53 



profound changes in the positions of the vessels are thus brought about, for the 

 vessels must shift their positions with the organs which they supply. 



Ventral Dissections. — Ventral dissections of the viscera are very easily 

 made. With the safety razor blade, start a cut in a coronal plane through 

 the caudal end of the embryo and the lower limb buds (Fig. 143). Extend this 

 cut laterad and cephalad through the body wall and the upper limb bud. The 

 head may be cut away in the same plane of section and the cut continued through 

 the body wall and upper limb bud of the opposite side back caudally to the start- 

 ing point. Section the embryo in a coronal plane, parallel with the first section 

 and near the back, so that the embryo will rest upon the flattened surface. With 

 forceps, now remove the ventral body wall. By tearing open the wall of the 

 umbilical cord along one side it may be removed, leaving the intestinal loop in- 

 tact. Pull away the 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. Favor- 

 able sections through the caudal end of the body may show the urogenital sinus, 

 rectum and sections of the umbilical arteries and allantois (Figs. 92, 119 and 143). 

 In late stages, by removing the digestive organs the urogenital ducts and glands 

 are beautifully demonstrated (Figs. 216 and 217). 



DEVELOPMENT OF THE FACE 



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

 opment of the face. The heads should be removed by passing the razor blade 

 between the heart and adjacent surface of the head, severing the neck. Next 

 cut away the dorsal part 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. 

 92 and 144 A and B. 



In the early stages (Figs. 92 and 119) the four branchial arches and clefts 

 are seen. The third and fourth arches soon sink into the cervical sinus, while 

 the mandibular processes of the first arch are fused early to form the lower jaw. 

 The frontal process of the head is early divided into lateral and median nasal 

 processes by the development of the olfactory pits. The processes are distinct 



