66 Devclopincjit and .Shape of Uriiiii'erous Tubules 



proximal convoluted portions, so that this portion forms a prominent part 

 of the entire tubule for a certain period of its development, as may be 

 seen in reconstructions and in sections. The epithelium lining this portion 

 of the tubule as this elongates differentiates into one showing cells with 

 clear protoplasm and nuclei in basal position as described for the proxi- 

 mal convoluted tubule. The loop of Henle of these tubules for a certain 

 period of their development remains relatively short, when their length 

 is compared with that of the proximal convoluted portion. These tubules 

 generally show throughout, but especially in their proximal convoluted 

 portions, a greater diameter than tubules which develop later, the lumen 

 of the proximal convoluted portion being especially wide. Their Mal- 

 pighian corpuscles are also relatively large. In tubules which develop 

 from renal vesicles that follow the first few generations of these, the loop 

 of Henle elongates at a relatively early period of their development so as 

 to form a prominent portion of the tubule at a time when the proximal 

 convoluted portion shows only a few relatively short coils; the proximal 

 and distal convoluted portions taken together form thus a smaller mass 

 than is formed by similar portions of the tubule tirst formed at a time 

 when each type presents a loop of Henle of about the same length. This 

 gives the two types of tubules when seen in reconstructions a characteristic 

 form, and enables a distinction between them. To characterize these 

 differences of form more clearly, reference is made to the models repro- 

 duced in Figs. 14 and 16. In C and D of Fig. 14, are shown two tubules 

 reconstructed from the kidney of a rabbit embryo measuring 3.5 cm. 

 They are from the layer of tubules situated nearest the pelvis of the 

 kidney and are therefore of those which are first formed. Attention is 

 called to the length and thickness of the proximal convoluted portion of 

 each of these tubules and to the relative shortness of the loop of Henle. 

 In A and B, are shown two tubules, presenting different stages of devel- 

 opment, reconstructed from the same kidney and are representative of 

 tubules the Malpighian corpuscles of which are situated nearer the peri- 

 phery of the kidney and were therefore differentiated later than the 

 Malpighian corpuscles and tubules of which C and D are types. In the 

 second type of tubules, the loop of Henle is relatively long, when com- 

 pared with the proximal convoluted portion; in A, of about the same 

 length as the loop of tubules C and D, while the proximal convoluted 

 portion of tubule D measures 1.5 mm., of C, 1 mm., and of A only 0.5 

 mm. The difference in shape presented by the two types of tubules is 

 apparent from the figures. The Malpighian corpuscles and coiled por- 

 tions of the uriniferous tubules situated nearest the pelvis of the kidney 

 form a distinct layer, just above the mesenchyme which surrounds the 



