36 WILLIAM MORTON WHEELER, 



It becomes more pronounced as development progresses as shown in 

 the next stage (Fig. 55). Here the whole structure is very much 

 larger, and the portion (mes.f) marked off from the thickened peri- 

 toneum is somewhat globose. The opposite end of the cell-cord ap- 

 pears to be wedging itself in between the cells of the pronephric duct. 

 A great advance is made in the next stage (Fig. 56). The two ends 

 of the cell-cord, the one next to the peritoneum and the other which 

 has pierced the wall of the pronephric duct, have remained fixed 

 while the whole tubule has been lengthening. This has caused it 

 to become U-shaped. The greatest growth in length takes place at 

 the end next to the duct, where the cell-strand retains essentially the 

 characters which it had in the earlier stage of Fig. 54. The other 

 limb of the U, however, has acquired a lumen by the moving apart 

 of its component cells. This lumen is closed by a plate of cells which 

 is applied to, although sharply marked off from the thickened peri- 

 toneum. A still older stage is shown in Fig. 58. Here the connection 

 of the tubule with the duct is not shown because the greater con- 

 volutions of the former do not fall within the plane of the section. 

 Between the truncated, closed end of the tubule and the peritoneum 

 the glomerulus is forming (mes. gl). The end of the tubule is moved 

 away from the peritoneum in which it originated, and cavities, in this 

 section two, each containing a blood-corpuscle, are the fore-runners of 

 the glomerular capillaries. Finally, in stage 57, the definitive parts 

 of the mesonephric tubule are all distinctly recognizable. It has in- 

 creased greatly in length and has become much contorted so that it 

 is cut in several places. The lumen of the bhnd terminal end next 

 to the peritoneum becomes very large and the cells of its wall dif- 

 ferentiate in two directions. Those forming the truncated closed 

 portion of an earlier stage are flattening out to form an investment 

 (Bowman's capsule) for the glomerulus, while the other cells arrange 

 themselves very regularly to make the funnel and each cell acquires a 

 tiagellum. The glomerulus arises from the large pale retroperitoneal 

 cells which form an ingrowth containing cavities, the primitive lumina of 

 the glomerular capillaries. Blood corpuscles and plasma enter these 

 cavities and the circulation is soon established. The thickened peri- 

 toneum which gave rise to the tubule in a much younger stage, re- 

 sumes its original character, flattening out so that it no longer differs 

 from the general peritoneum investing the uephric lobe. By this time 

 pigment has begun to be deposited. The appearance of the meso- 



