TRANSVERSE SECTIONS OF A SIX MM. PIG EMBRYO 



III 



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 coelom near the midline. 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 sec- 

 tion 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 cutis 

 plate, lying just beneath the ectoderm. This plate curves 

 lateral to the spindle-shaped muscle plate which gives rise to 

 the voluntary muscle. Next comes a diffuse mass of mesen- 

 chyma, the sclerotome, which eventually, with its feUow of the 



Spinal ganglioir 

 Interscgmeiiliil^ 

 artery 

 Muscle plate 



Cutis plate 



Sclerotome 



Ectoderm 



Spinal cord 



R. umbilical v 



R. umbilical artery 

 Tail 



Mesonephric duct 



L. umbilical 

 artery 



L. umbilical vein 



Allantois and cloaca 



L. umbilical artery 

 Cwlom 



Notochord 



Spin il cord' 



Fig. 116. — Transverse section through the umbilical vessels, allan- 

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



Fig. 115. — Transverse sec- 

 tion through the primitive seg- 

 ments and spinal cord of a 6 mm. 

 pig embryo. X 45. 



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). — Hav- 

 ing 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 allantoic stalk, curving in- 

 ward 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 at the left of the figure. 



