148 A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



pronephros. The bulbous portion is the inner pronephric chamber. It is 

 connected to the mesothelium of the ccelomic cavity by a slender strand of cells, 

 the nephrostome, which joins it ventrally. The caudal portion of the tubule 

 Felix terms the principal collecting tubule. 



The third pronephric tubule is the longest. It is rounded in cross-section 

 and more regular in form than any of the succeeding tubules. The inner pro- 

 nephric chamber is small cephalically, but is not connected to the mesodermal lining 

 of the body-cavity by means of a nephrostome. A pit on the mesodermal lining 

 of the body-cavity opposite the tubule, however, probably represents a broken- 

 down nephrostome. The principal collecting tubule extends caudally close to the 

 ectoderm. It terminates in relation with the principal collecting tubule of the 

 fourth pronephric tubule. Whether an actual fusion exists between the two 

 collecting tubules or whether they merely touch, I am unable to determine. The 

 third pronephric tubule possesses a distinct lumen, which extends throughout 

 most of its length. It lies in the tenth body segment, its collecting tubule extend- 

 ing into the eleventh segment. 



The fourth tubule is almost the same shape and size as the third and is less 

 regular in form. Again a connecting nephrostome is lacking, although its position 

 is marked on the free surface of the mesothelium by a depression. The principal 

 collecting tubule ends in connection with that of the fifth tubule. The joined 

 distal ends of the collecting tubules give rise to the primary excretory duct. The 

 fourth tubule possesses a discontinuous lumen divided into two portions. 



The fifth (plate 4, fig. 2), sixth, and seventh tubules differ from the third and 

 fourth in possessing connecting nephrostomes. Each nephrostome begins as a 

 funnel-shaped opening on the coelomic wall. In no case, however, can a lumen 

 be traced into the tubule. Each nephrostome is joined to its inner pronephric 

 chamber by a strand of cells. Of these, that of the fifth tubule is extremely long 

 and band-like. The inner pronephric chambers are definite swellings situated 

 near the middle of the tubule. That of the seventh is the largest. The principal 

 collecting tubules are joined by the preceding ones as shown in plate 4, figure 4. 

 Lumens are present only in the inner pronephric chambers. The fifth and sixth 

 pronephric tubules are situated in the twelfth body-segment, while the seventh 

 is found in the thirteenth. 



Histologically the tubules in all parts are made up of polygonal or columnar 

 cells, with rounded or elongated nuclei. Where a lumen is present the cells are 

 arranged radially about it, but in other places there seems to be no definite 

 arrangement. 



Beyond the seventh pronephric tubule and apparently continuous with it is 

 a large rounded cord of cells which extends through the remainder of the embryo. 

 This cord possesses a number of irregular swellings, the mesonephric vesicles. 

 These vary in size and shape and are not so definitely marked off from one another 

 as shown by Felix 10 and Watt 48 ; 16 to 18 may be counted (plate 4, fig. 4); they 

 possess nephrostomes connecting them with the ccelomic mesothelium similar to 

 those of the pronephric tubules. 



