KIDNEY IN RELATION TO WOLFFIAN BODY IN THE CHICK. 161 
backwards or for^vards, the dorsal dilatation w^ould be no 
longer visible, but occupying the position of the dorsal part 
of the dilatation, would be seen a tubule surrounded by the 
cells of the kidney blastema (fig. 23). 
In the next section to this the walls of the tubule become 
indistinctly marked off from the kidney blastema (fig. 24). 
Some of the large columnar cells of the kidney tubule become 
branched, the processes being continuous with the processes 
of the branched cells of the kidney blastema. In fact, every 
stage of cell shape between a columnar lining cell of the 
tubule and a branched cell of the blastema is visible. 
The lumen of the tubule is no longer distinct, it not being 
possible to say what is an intercellular space and what the 
lumen of the tuhule. 
In the next section no trace of a tubule is visible, its 
place being occupied by the cells of the blastema. 
Fig. 23 is taken from a section next but one to fig. 22. 
Nine such dorsal dilatations of the ureter, with commencing 
tubules, growing from them, could be made out in the embryo 
under consideration. 
In front of the most anterior the tubules open directly into 
the ureter which in this region has become more dorsal with 
regard to the Wolffian body. Tubules in this region, more- 
over, are given ofi* from the ventral side of the ureter, corre- 
sponding almost exactly to those given off from the dorsal side. 
Four pairs could be made out ; after which the ureter ended 
closely surrounded on all sides hy dense kidney blastema. 
The next stage, which I have closely examined, w-as in an 
eight-day chick. The kidney had reached a great compli- 
cation of structure. Malpighian bodies had, however^ not 
distinctly appeared. The tubules were still surrounded by 
kidney blastema which was especially conspicuous at their 
growing ends. The appearance of the latter, which was ex- 
actly similar in all essential details to the growing points of 
the tubule last described, is represented in fig. 24. 
Before considering the bearing of the above facts upon the 
questions asked at the outset, I will recapitulate the more 
important points in the development of the Avian kidney 
and Wolffian body. 
1. The cells which give rise to the Wolffian and kidney 
tubules do not develop as involutions of the peritoneal epi- 
thelium, hut from a blastema of cells derived from the inter- 
mediate cell mass. 
2. The blastema of the kidney is at first perfectly con- 
tinuous with that of the Wolffian body, and cannot be dis- 
tinguished from it. 
