TRANSVERSE SECTIONS OF A SIX MM. PIG EMBRYO 



body walls, ventral to the mesonephros, occur the umbilical veins. The left vein is large 

 and cut lengthwise. The right vein is cut obliquely twice. 



In the ventral portion of the section, the lower limb buds are prominent laterally. A 

 large pair of arteries, the common iliacs, are given off from the aorta and may be traced 

 into connection with the umbilical arteries. The large intestine, supported by a short 

 mesentery, lies in the ccelom near the midplane. On each side are the mesonephric folds, 

 here small and each showing a section of the mesonephric duct and a single vesicular anlage 

 of the mesonephric tubules. The mesonephric ducts are sectioned as they curve around 

 from their position in the dorsal portion of the section. 



Section through the Primitive Segments and Spinal Cord (Fig. 115). This section 

 is near the end of the series, and, as the body is here curved, it is really a frontal section. 

 At the left side of the spinal cord, the oval cellular masses are the spinal ganglia cut across. 

 The ectoderm, arching over the segments, indicates their 

 position. Each segment shows an outer dense layer, the 

 dermatome, lying just beneath the ectoderm. This plate 

 curves lateral to the spindle-shaped myotome, which gives 

 rise to the voluntary muscle. Next comes a diffuse mass 

 of mesenchyma, the sclerotome, which eventually, with its 



Spinal ganglion 

 Intersegmental. 

 artery 

 Myotom 



Dermatim 



Sderol 



Ectoderm 

 Spinal cord 



FIG. 115. Transverse sec- 

 tion through the primitive 

 segments and spinal cord of a 

 6 mm. pig embryo. X 45. 



R. umbilical vein 



R. umbilical artery. 

 Tail 



Mesonephric duct 



Spinal cord 



L. umbilical vein 

 Allantois and cloaca 



L. umbilical artery 

 Ccelom 



Notochord 



FIG. 116. Transverse section through the umbilical 

 vessels, allantois and cloaca of a 6 mm. pig embryo. 

 X 45- 



fellow of the opposite side, surrounds the spinal cord and forms the anlage of a vertebra. 

 A pair of spinal nerves and spinal ganglia are developed opposite each somite, and pairs 

 of small vessels are seen between the segments. These are dorsal intersegmental arteries. 

 Section through the Umbilical Vessels, Allantois and Cloaca (Fig. 116). Having 

 now studied sections at various levels to near the end of the series, we shall next examine 

 sections through the caudal region and study the anlages of the urogenital organs. Owing 

 to the curvature of the embryo, it is necessary to go cephalad in our series. The first 

 section passes through the bases of the limb buds at the level where the attantoic stalk, 

 curving inward from the umbilical cord, opens into the cloaca. At either side of the allan- 

 toic stalk may be seen oblique sections of the umbilical arteries, and lateral to these the 

 large left and small right umbilical vein. The mesonephric ducts occupy the mesonephric 

 ridges which project into small caudal prolongations of the ccelom. Midway between the 



