74 ADAM SEDGWICK. 



behind this point. But it does not all undergo the above 

 development into Wolffian tubules. It breaks up into 

 Wolffian tubules as far back as the thirtieth segment. Behind 

 this pointj i. e. from tlie thirty-first to the thirty-fourth 

 segments inclusively, the Wolffian blastema undergoes quite 

 a different fate. It remains for some time almost quite pas- 

 sive and ultimately gives rise to the epithelium of the per- 

 manent kidneys. In consequence of this I have called that 

 part of the Wolffian blastema between the thirty-first and 

 thirty- fourth segments the kidney blastema; and in future 

 shall refer to it by that name (figs. 12, 15, 16, 17, kb). It 

 is important to notice that this kidney blastema develops 

 in an exactly similar manner to the Wolffian blastema. 



It is not until well into the fourth day, when the ureter 

 has appeared, that it is possible to draw the line between 

 the two. 



Fig. 12 is taken from the thirty-second segment of a chick 

 with thirty-four protovertebrse ; it shows a blastema of cells 

 lying just internal to the Wolffian duct. Fig. 10 is taken 

 from the twenty-ninth segment of the same chick. It shows 

 the hindermost trace of a Wolffian tubule I could find at 

 this stage. In all the sections between figs. 10 and 12 there 

 is present, just as in fig. 12, a blastema of cells lying just 

 internal to the Wolffian duct. 



In a slightly older embryo the hindermost trace of a 

 Wolffian tubule would be in the thirtieth segment. In still 

 older embryos secondary tubules would have appeared in the 

 thirtieth segment, but no trace of a primary tubule in the 

 thirty-first, and so on in later stages, Wolffian tubules never 

 appearing in the thirty-first segment. In the embryo, from 

 which figs. 13 to 17 were taken, the ureter had not appeared. 

 In examining a series of sections from the posterior part of 

 this embryo, some of which are figured (figs. 13 to 17), the 

 following points are noticeable, illustrating what has just 

 been stated. 



A primary and secondary tubule are present in fig. 14, and 

 it is almost the last section in which any trace of a Wolffian 

 tubule can be seen (the two above tubules are cut in the 

 next two sections). The tubules adjacent anteriorly to these 

 are three in number (fig. 13), consisting of primary, secon- 

 dary, and tertiary. Supposing the Wolffian body were going 

 on developing in the region behind that from which fig. 14 

 was taken, we ought at the least to find at this stage primary 

 tubules in that region, for the formation of primary and 

 secondary tubules is always separated by an interval of time. 

 But no such primary tubules are seen. Behind fig. 14 (figs. 



