TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 287 



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consists of this epithelium and of the 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 diagrammatically, 

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

 that the abdominal cavity (splanchnocele) 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 fetal or 

 embryonic kidney, and is also termed the mespnephros (compare page 109). Rela- 

 tively to other parts, it is much larger 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 Wolffian 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 glomcrv-'us becomes 

 continuous with the covering of the glomerulus itself, the whole structure resem- 

 bling closely that of a Malpighian corpuscle of the true kidne". The space around 

 each glomerulus is really the beginning of a Wolffian tubule The spaces between 

 the tubules are almost entirely blood-channels, and are lined by enddtKeliui. 

 for the most part, is closely fitted against the epithelium of the tul lies. Ov_ 

 ally a small amount of mesenchyma can be found between the tubules, or 

 the tubules and the nearest endothelium. We have, accordingly, in tilts-,- <>; 

 a. typical sinusoidal circulation. The blood spaces of the Wolffian body really 

 belong to the posterior cardinal veins into which the Wolffian tubules in the course 

 of their development have, as it were, penetrated, although without destroying the 

 continuity of the vascular endothelium It is by the intercrescencf of the tubules 

 and of the endothelium that the sinusoidal condition is establish^- 

 the original channel remains on the dorsal side of the Wolffiar mu. 



less free, V '.card. We thus learn that, owing to the developmen of ilu- Wolffian 

 body, the posterior cardinal veins as such disappear. The Wolffian di, 

 on the ventral side of the organ, and can easily be traced through as a continuous 

 tube from section to section. In the figure it may be easily found in the left 

 mesonephros, it being there the lowermost of the cavities drawn in the organ. 

 On the median lower surface of the Wolffian body, underneath the glomeruli, 

 is an accumulation of tissue. Gen, the anlage of the genital gland, which is yet 

 very slightly advanced. Below the " -n HUM, side of the embryo is a 



