432 FREDERIC T. LEWIS 



For permission to make and publish figure 12, the writer is 

 under special obligation to Dr. Frank H. Rose, who made the 

 model, and to Prof. Franklin P. Johnson, who directed Doctor 

 Rose's study in the University of Missouri. This model of a 

 tubule in a 20-mm. pig is incidental to a comprehensive study of 

 that embryo, being made by Professor Johnson and his pupils, 

 and later to be published in detail. The simplified sketch, show- 

 ing all the essential bends and loops in this long and involved 

 tubule, may readily be compared with the figures of the earlier 

 stage already discussed. From the glomerular capsule, with 

 initial kinks, the portion C sweeps in a well-rounded curve to 

 the dorsal border, and forms a U in the concavity of the C and 

 a V in the concavity of the U, with a final sweeping curve enclos- 

 ing them all — the portion Z. 



This model again accords, in surprising detail, with Mac- 

 Callum's figure of the tubule from a much larger pig, measuring 

 80 mm. Neither the length of the tubule nor the magnification 

 of the figure is stated, but the tubule, here shown reversed in 

 figure 13, is evidently much longer than in the preceding stage. 

 A secondary coil has appeared along the portion U and another 

 in connection with V. The coil in the descending limb of the 

 U chances to be comparable with a kink in figure 12, but the con- 

 stancy of this feature remains to be determined. The new loop 

 in segment V, which may or may not correspond with a slight 

 bend in the tubule at 20 mm., is not directed toward the apex 

 of the V; if it should become so, it would carry the evolution of 

 the characteristic pattern one step further than has yet been 

 observed. 



Finally, it may be repeated that the total number of tubules 

 studied is small for drawing general deductions. But the close 

 agreement in the findings of independent observers and the 

 absence of a single aberrant form from the group carefully mod- 

 eled are evidence in favor of the interpretation presented. In 

 these models an easily recognized pattern exists, much more 

 complicated than the double-spiral form from which it springs. 



