I40 THE DISSECTION OF PIG EMBRYOS 



Lateral dissections of embryos 18 mm. and 35 mm. long show infinitely better 

 than sections the form and relations of the organs, their relative growth, and 

 their change of position (Figs. 144 and 145). Compare the organs of 6, 10, 18, 

 and 35 mm. embryos and note the rapid growth of the viscera (see Figs. 95 and 

 120). Hand-in-hand with the increased size of the viscera goes the diminution 

 of the dorsal and cervical flexures. In the brain, note the increased size of the 

 cerebral hemispheres of the telencephalon and the presence of the olfactory lobe of 

 the rhinencephalon. The cerebellum also becomes prominent and a ventral 

 flexure in the region of the pons, the pontine flexure, is more marked. The 

 brain grows relatively faster than the spinal cord, and, by the elongation of 

 their dorsal roots, the spinal ganglia are carried ventral to the cord. The body 

 of the embryo also grows faster than the spinal cord, so that the spinal nerves, 

 at first directed at right angles to the cord, course obHquely caudad in the 

 lumbo-sacral region. 



Median Sagittal Dissections (Figs. 146 and 147). — Preliminary to the dissection, a 

 cut is made dorsally as near as possible to the median sagittal plane. Beginning caudally at 

 the mid-dorsal line, an incision is started which extends in depth through the neural tube and- 

 the anlages of the vertebra;. 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 caudally, cutting through the heart and hver to the right oj the midline and oj 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 puUing 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 struc- 

 tures intact. If the plane of section was accurate, the brain and spinal cord wiU 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 overhe the stomach 

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

 portion of the hver until the hepatic diverticulum is laid bare. It is whitish in color and may 

 thus be distinguished from the brownish hver. Beginning at the base of the umbilical cord, 

 carefully pull away its right wall with forceps, thus exposing the intestinal loop and its attach- 

 ment to the yolk stalk. If in the caudal portion of the umbilical cord the umbihcal 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 lejt 

 mesonephros 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 surrounding mesenchyma. With a few trials, such dissections may be made in 



