MEDIAN SAGITTAL DISSECTIONS 



149 



depth through the neural tube and the anlages of the vertebrae. This incision is 

 carried to the cervical flexure, cranial to which point the head and brain are 

 halved as accurately as possible. The blade is then carried ventrally and cau- 

 dally, cutting through the heart and liver to the right of the midline and of the 

 umbilical cord until the starting point is reached. A parasagittal section is next 

 made well to the left of the median sagittal plane and the sectioned portion is 

 removed, leaving on the left side of the embryo a plane surface. With the embryo 

 resting upon this flat surface, the dissection is begun by removing with forceps 

 the right half of the head. In pulling this away caudalwards, half of the dorsal 

 body wall, the whole of the lateral body wall, and the parts of the heart and liver 

 lying to the right of the midline will be removed, leaving the other structures in- 

 tact. If the plane of section was accurate, the brain and spinal cord will be 

 halved in the median sagittal plane. Wash out the cavities of the brain with a 

 pipette and its internal structure may be seen. Dissect away the mesenchyma 

 between the esophagus and trachea and expose the lung. Remove the right 

 mesonephros, leaving the proximal part of its duct attached to the urogenital 

 sinus. The right dorsal lobe of the liver will overlie the stomach and pancreas. 

 Pick it away with forceps and expose these organs. Dissect away the caudal 

 portion of the liver until the hepatic diverticulum is laid bare. It is whitish in 

 color and may thus be distinguished from the brownish liver. Beginning at the 

 base of the umbilical cord, carefully pull away its right wall with forceps, thus 

 exposing the intestinal loop and its attachment to the yolk stalk. If in the caudal 

 portion of the umbilical cord the umbilical artery is removed, the allantoic stalk 

 may be dissected out. To see the anlage of the genital gland, break through and 

 remove a part of the mesentery, exposing the mesial surface of the left meso- 

 nephros and the genital fold. The dissection of the metanephros and ureter is 

 difficult in small embryos. In 10 to 12 mm. embryos, the umbilical artery, just 

 after it leaves the aorta, passes lateral to the metanephros and thus locates it. 

 By working carefully with fine needles the surface of the metanephros may be 

 laid bare and the delicate ureter may be traced to the base of the mesonephric 

 duct. The extent of the dorsal aorta may also be seen by removing the surround- 

 ing mesenchyma. With a few trials, such dissections may be made in a short 

 time, and are invaluable in giving one an idea of the form, positions and rela- 

 tions of the different organs. By comparing the early (Figs. 91 and 117) with 

 the later stages (Figs. 141 and 142) a number of interesting points may be noted: 

 In the brain, the corpus striatum develops in the floor of the cerebral hemi- 

 spheres. The interventricular foramen is narrowed to a slit. In the roof of the 



