TRANSVERSE SECTIONS 



119 



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 midline. On each side are the mesonephric folds, here small and each show- 

 ing 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. no). This 

 section is near the end of the series and as the body is here curved it is really a longitudinal 

 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 cutis plate, lying just be- 

 neath the ectoderm. This plate curves lateral to the 

 spindle shaped muscle plate which gives rise to the volun- 

 tary muscle. Next conies a diffuse mass of mesenchyma, 

 the sclerotome, which, eventually, with its fellow of the 

 opposite side, surrounds the spinal cord and forms the 



/f. A Ml BE *ft ^\ 

 Spinal gang. 

 Interseg- 

 mental artery 

 Muscle plate 



Cutis plate 



Sclerotome 



Ectoderm 



Spinal cord 



FIG. no. Transverse sec- 

 tion through the primitive seg- 

 ments and spinal cord of a 6 

 mm. pig embryo. X 45. 



Vein 



R.umbiHcol artery 

 Jail 



Mesonephric A 



Spinal cord 



Notochord 



FIG. in . Transverse section through the umbilical vessels, allan- 

 tois and cloaca of a 6 mm. pig embryo. X 45. 



anlage of a vertebra. From it is developed also connective tissue. 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. in). 

 We have now studied sections at various levels of the 6 mm. embryo to near the end of the series. 

 We shall next examine sections through the caudal region and study the anlages of the uro- 

 genital organs. Owing to the curvature of the embryo, we will now be going cephalad in our 

 series. The first section passes through the bases of the limb buds at the level where the 

 allantoic stalk, curving inward from the umbilical cord, opens into the cloaca. At either side 

 of the allantoic 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 

 ducts lies the hind-gut, dorsal to the cloaca. The tip of the tail is seen in section to the 

 left of the figure. 



