142 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 in- 

 creased 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 pres- 

 ence of the olfactory lobe of the rhinencephalon. The cerebellum also becomes prominent 

 and a ventral flexure in the region of the pons, the p online 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 obliquely 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 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 caudally, cutting through the heart and liver to the right of the mid- 

 plane and 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 midplane will be 

 removed, leaving the other structures intact. 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 mesen- 

 chyma between the esophagus and trachea and expose the lung. Remove the right meso- 

 nephros, 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 the umbilical artery is removed in the caudal portion of the umbilical cord, 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 mesonephros and 

 the genital fold. The dissection of the metanephros and ureter is difficult in small em- 

 bryos. 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 are made in a short time ; 

 they are invaluable in giving an idea of the form, positions, and relations of the different 

 organs. By comparing the early (Figs. 96 and 122) with the later stages (Figs. 146 and 

 i47)~a number of interesting points may be noted. 



In the brain, the corpus striatum develops in the floor of the cerebral hemispheres. 

 The inter-ventricular foramen is narrowed to a slit. In the roof of the diencephalon appears 

 the anlage of the epiphysis, or pineal gland, and the chorioid plexus of the third ventricle. 



