202 STUDY OF PIG EMBRYOS. 



of a vertebra, which appears as an area relatively large, over which the mesen- 

 chymal cells are more crowded or condensed than elsewhere. At its periphery the 

 anlage merges without divisional boundary into the surrounding mesenchyma. 

 It is more expanded laterally than ventrally. In the median line below the 

 notochord is the large dorsal aorta, Ao, which is formed by the union of the two 

 descending aortse shown in figure 120, and which extends through the abdominal 

 region of the embryo to the pelvic region, where it forks to form the two allantoic 

 arteries, which, passing on either side of the intestine, continue their course along 

 the side of the internal allantois or future bladder, until they reach the umbilicus, 

 where they enter the umbilical cord to supply the extra-embryonic or placental 

 circulation. The aorta is surrounded by mesenchyma, and to this are, so to 

 speak, appended the large Wolffian bodies, W . B, one on each side. From the 

 dorsal region of the embryo to the umbilical cord extends the somatopleure or 

 body-wall, Som, which, like that around the pericardial chamber, consists of an 

 external ectoderm, Ec, a middle mesenchyma, mes, and an internal mesothelium, 

 tnsth. It is important for the student to understand the arrangement of the 

 germ-layers in the somatopleure. The mesothelium is commonly known in the 

 descriptive anatomy of the adult as the peritoneal epithelium. The peritoneal 

 membrane consists of this epithelium and of all underlying connective tissue. In 

 sections like that figured it can be readily followed not only over the inner surface 

 of the body-wall, but over the surface of the Wolffian body and liver, and upon 

 the left side of the body also over the surfaces of the greater omentum, stomach, 

 and lesser omentum. The relations of the abdominal viscera to the peritoneum 

 which are so perplexing to the student of adult anatomy are here shown diagram- 

 matically, as it were, by the section of the actual embryo. It is evident from 

 such a section that the abdominal cavity (splanchnoeele) is completely bounded 

 by mesothelium, and that all the abdominal viscera are outside of the cavity. 

 This conception, which is so important, yet so difficult to the student of anatomy, 

 is easily mastered by the study of embryonic relations. The Wolffian body, 

 W. B, is the foetal or embryonic kidney, and is also termed the mesonephros (com- 

 pare page 101). It is much larger relatively to other parts in the pig than in man 

 or the rabbit. It consists of numerous epithelial tubules very much contorted 

 with blood spaces between them, of glomeruli which always lie toward the median 

 and inferior side of the organ, and, finally, of a single longitudinal canal, the Wolff- 

 ian duct, into which all of the tubules open. The tubules are formed by the 

 cuboidal epithelium. The glomeruli resemble in their structure those of the 

 kidney. Each is a bunch of blood-vessels covered in by a layer of epithelium 

 which forms one boundary of the space into which the glomerulus projects. The 

 opposite side of the space is also bounded by epithelium, which at the stalk of the 



