440 TEXT-BOOK OF EMBRYOLOGY. 



of 5 to 6 weeks (about 20 mm.) or in pig embryos of 25 to 35 mm. Fix the pig embryos, 

 for example, in Zenker's fluid or Bouin's fluid, cut transverse sections in paraffin and stain 

 with Weigert's haematoxylin and eosin. Serial sections are, of course, necessary if one 

 wishes to trace the various structures or to make reconstructions. The mesonephroi at this 

 stage extend from the level of the stomach to the pelvic region and fill a considerable portion 

 of the abdominal cavity. The mesonephric duct lies in the peripheral part of the organ, 

 forming a rather wide tubule; the Miillerian duct lies near by, forming a much smaller 

 tubule; the two ducts in the later stages are embedded in an elevation on the surface of the 

 organ. On the medial side of the mesonephros a distinct projection constitutes the genital 

 gland. 



The Kidney. Fix a pig embryo of about 17 mm. in Zenker's fluid or Bouin's fluid. 

 Unless the embryo is to be used for the study of other structures in the anterior region, 

 remove the anterior half. Cut transverse serial sections of the posterior half in paraffin 

 and stain with Weigert's haematoxylin and eosin. The anlagen of the kidneys are found 

 at the level of the caudal ends of the mesonephroi. They are situated lateral to the aorta 

 and are composed of irregular tubules surrounded by dense mesenchymal tissue. The 

 tubules represent the straight renal tubules which have grown out from the renal pelvis; 

 the latter is usually a large, more centrally located space. The dense mesenchymal tissue is 

 the nephrogenic tissue (metanephric blastema) which gives rise to the convoluted renal 

 tubules. 



By following the series of sections caudally, the renal pelvis can be traced to the ureter 

 and the latter to its opening into the mesonephric duct. 



Further development of the kidney can be studied in sections of older embryos, prepared 

 by the same technic as those used for the earlier stages (see above). In more advanced 

 stages the anlagen of the convoluted renal tubules are seen as very dense portions of the 

 mesenchymal tissue, lying in contact with the straight tubules. The cells become epithelial 

 in character and join those of the straight tubules. 



Wax reconstructions are valuable adjuncts in the study of the kidney. One should be 

 made of a kidney and ureter at an early stage (pig embryo of 17 mm.) and another of a 

 group of renal tubules at some later stage. 



The Bladder, etc. It is very difficult, by studying sections alone, to get a comprehensive 

 view of the changes that occur in the bladder, in the proximal ends of the ureters and meso- 

 nephric ducts, and in the urethra. Some idea of the interrelation of these structures may be 

 gained by tracing, in serial sections of a 17 mm. pig embryo, the ureter to the mesonephric 

 duct, the latter to the urogenital sinus, and the urogenital sinus in one direction to the exterior 

 and in the other direction to the urachus. Prepare sections as under "The Kidney" (see 

 above). At the same time the openings of the Miillerian ducts on the dorsal side of the uro- 

 genital sinus should be noted, and the ducts traced forward through the genital cord, in 

 company with the mesonephric ducts, and along the surface of the mesonephroi. A wax 

 reconstruction of all these parts is very instructive. 



In larger embryos, the various structures can be identified by very careful dissection. 

 Dissections of human embryos of two months or more are especially valuable. 



The Genital Ridge. A single stage in the development of the genital ridge serves to 

 indicate its origin from the mesothelium and its primary location. Fix a pig embryo of 10 

 to 12 mm. in Flemming's fluid, making an incision in the abdominal wall to give the fixa- 

 tive access to the interior. Cut transverse sections in paraffin through the embryo at the 

 level of the lower part of the liver, and stain with Heidenhain's haematoxylin. The genital 

 ridge is found on the medial side of the mesonephros and is composed of small, dark cells 

 and larger, clearer sex cells, 



