WOLFFIAN TUBULES IN MAMMALIAN EMBRYOS 431 



1909 by Dr. F. T. Krusen, in connection with the undergraduate 

 course in embryology given by Professor Minot and assistants. 

 This model, duly catalogued and placed in the Harvard Collection, 

 has apparently never before been utilized. Its essential features 

 are shown in figure 10, in which such slight displacements have 

 been made as are necessary for following the coils easily. In 

 this tubule, from a 6-mm. pig, the segment U is directed toward 

 the concavity of C, and a new loop, V, intervening between U 

 and Z, has appeared, pointed toward the concavity of the U; 

 Z is without special features. This pattern might be regarded 

 as a casual form were it not duplicated in MacCallum's figure, 

 here reproduced reversed as figure 11. 



MacCallum described his figure as a "diagrammatic recon- 

 struction of Wolffian tubules" from a pig embryo of 8 mm., and 

 he presented it as applying to "a number of tubules." Referring 

 to an older embryo, he writes: "Special names might be given 

 to the different parts of the tubule, but until their significance 

 is more definitely known this could be of little value. There is, 

 however, a very distinct division into a secretory and a conduct- 

 ing part." But without discussing relative values of morpho- 

 logical and physiological subdivisions, it is clear that we stand 

 on firmer ground in describing the course of the tubule than in 

 assigning a functional significance to its various bends. It is 

 unfortunately true that the limits of the secretory portion in 

 the pig remain unknown and MacCallum did not succeed in 

 defining them. He put together the upper part of C (in fig. 11) 

 and the adjacent limb of the U as the 'secretory loop/ and this 

 loop forms the only subdivision of the tubule which he recognized. 

 But judged by its large diameter in figure 10, the remainder of 

 the U is also secretory; whereas in man none of the part U was 

 of the same nature as C, and the latter was certainly secretory. 

 Whether the subdivisions here imposed upon MacCallum's figure 

 are profitable or not may be questioned, but they enable one to 

 sketch quickly the course of the Wolffian tubule not only in the 

 young embryos now considered, but also in older ones where 

 previously no description was attempted. 



