ADULT HEAD KIDNEY OF BDELLOSTOMA STOUT [ 855 
from the seventeen or eighteen prmiary tubules of the embryo, 
one would expect to find about that number of maia stems in the 
adult through which all of the tubules would be connected with 
the central duct. But this is not the case. Occasionally such a 
stem is found, but usually instead of a single stem two or three 
tubules are seen opening into the duct together. This may be 
brought about in two ways: the main stem (which is never long) 
may in some way become so shortened that the early secondary 
tubules appear to open directly into the duct; then again, on 
account of the crowding together of the tubules in the embryo 
two primary tubules may come to open into the duct very close 
together. For these reasons it is difficult in a, series of sections 
to determine what must have been the number of primary tubules. 
And yet with care this may be done with considerable accuracy. 
In one case the duct was reconstructed on millimeter paper, and 
it was found that tubules connected with it at twenty places. This 
is about the number of primary tubules of both kinds found in 
the head kidney in the embryo. 
When the entire organ is viewed with a lens it presents a more or 
less distinctly lobed appearance (fig. 1), as has been observed by 
previous workers in other members of the group. Judging from 
what may be seen of the lobes and of the course of the tubules 
in a stained and cleared preparation, it seems probable that each 
lobe is made up of a single primary tubule, together with all of 
its branches. The number of distinct lobes, however, is usually 
not so great as one would expect if this were the case. 
The two chief causes for the increase in the size of the head kid- 
ney with age are the growth of the tubules in length and their 
increase in number; the diameter changes little if any. The fol- 
lowing figures will give an idea of the increase in length; in an 
individual 7.5 cm. inches long, in which only part of the tubules 
showed any indications of branching, the average length of a 
few of the primary tubules was 0.165mm. ; in an individual 27.9cm. 
long the average length of a few of the longest tubules was 
0.292mm.; in one 35.5 cm. long it was 0.464 mm.; and in one 
45.7 cm. long it was 0.90 mm. 
