CI. Carl FT liber . -31 



the aulage or the beginnins of the uriniferous tubules but more advanced stages 

 where union between tubular anlage and collecting tubule had already taken place 

 or that when earlier stages are represented as for instance by Disse (Fig. 77). lit 

 seems apparent that a continuity of the discontinuous structures is simulated by an 

 overlapping of the two structures. In sections passing through the epithelium of an 

 ampullar enlargement, without passing through its lumen and througli the inner zone 

 of the nephrogenic tissue and a renal vesicle, it often seems as if these structures were 

 continuous, presenting the appearance of a bud growing from the ampullar enlargement 

 of the primary collecting tubule. This is also true if the section passes obliquely 

 through these " structures. In such sections, especially if relatively thick, the inner 

 zone of the nephrogenic tissue, which, as has been stated, resembles epithelial tissue, 

 appears as a continuation of the epithelium of the collecting tubule. Ilaycraft makes 

 tlie statement that the most conclusive evidence in favor of the view that the epithe- 

 lium of the kidney tubules arises from tlie blastema " would be the presence of 

 isolated masses of ('ells and their subseo.uent .iunction with the tubules, but of this. 

 as we have seen, there is no proof." " What we have is an appearance at the tips 

 of the collecting tubules which suggests to the minds of many (;l)servers that the 

 cells of the blastema gradually transform themselves into epithelium." He further 

 states that such appearances are due to " nothing more nor less than oblique sections 

 of tubules, not through their extremities, but through bends in their course." To 

 this it may be answered that reconstructions show conclusively the presence of isolated 

 cell-masses — renal vesicles, the anlagen of uriniferous tubules — which subsequently 

 join the collecting tubules and further tliat the very argument which Ilaycraft uses 

 to point out the sources of error of those who hold to the discontinuous origin of the 

 uriniferous tubules from special cell masses may be used with equal pertinence to 

 explain the sources of error of observers who hold that the uriniferous tubules are 

 developed by a process of budding from the collecting tubules, namely that the 

 latter have" been led to interpret as continuous structures certain discontinuous 

 structures — primary collecting tubules, inner zone ff metanephrogenic tissue and 

 renal vesicles — which b.y reason of their close relation and a certain resemblance in 

 structure of tlieir constituent cells appear in oblique sections as forming a continuous 

 whole. My own observations, pertaining to human, cat. rabbit and pig embryos, as 

 may be apparent from statements here made, liave enabled me to confirm the observa- 

 tions of investigators who recognize a discontinuous origin for the uriniferous tubules 

 and who have described small vesicles, variously known as Bliischen, acini, or renal 

 vesicles, as the anlagen of the uriniferous tubules proper. 



Developmext of the Collecting Tubules axd Their Eelatiox to 



THE Eenal Vesicles. 



The further growth of the kidney is accomplished by a further 

 division of the branches of the primary renal pelvis and their differentia- 

 tion into the straight collecting tubnles, by a constant new formation of 

 renal vesicles and by a differentiation of the renal vesicle into the urin- 

 iferous tubules and their union with the collecting tubules. Each pri- 

 mary collecting tubule early divides, as has been stated, into two branches, 

 each of which shows a distal enlargement, an ainpulla, capped with a 

 layer of nephrogenic tissue. Each of these branches divides again into 

 two branches, the division beginning with the ampullar enlargement. With 

 the formation of this second series of branches, the zone of nephrogenic 

 tissue which surrounded the ampulla of each primary branch separates 

 into two parts, each of which surrounds the end of one of the resulting 

 branches. Several similar divisions follow in relatively quick succes- 

 sion and with the division of each end branch the nephrogenic tissue 

 which surrounds it separates into parts, which in each instance surround 

 the ends of the resulting branches. The end branches from each division 

 develop ampullar enlargements, which are found at the periphery of the 

 organs under the developing capsule, which enlarges as the kidney 

 develops. (In the developing kidney of human embryos, the end branches 

 are seen to end under the capsule and at the boundaries of the primary 



