ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA = 347 
organ and the changes which it undergoes in acquiring the state now 
described. 
The renal organs are plainly visible in buds not more than ~,th of 
an inch long as aggregations of clear, round, almost colourless 
corpuscles, between the atrial and the outer tunic. 
In describing the first stage of the bud (fig. 14), I have spoken of 
a thin layer of indifferent tissue which passed from the end of the 
endostylic cone, or prolongation, into the generative blastema. In 
more advanced stages, this tissue forms a sort of hood over the end of 
the saccular rudiment of the alimentary tract (figs. 15 and 16), and 
seems to be the means of connecting the end of that sac with the 
external tunic. 
After a time, however, a clear space appears around the apex of 
the sac, and separates this connecting mass from the rest, which now, 
consequently, appears as a broad zone (@) surrounding the sac below 
its apex, but above the uppermost of the branchial stigmata (fig. 19), 
This zone remains as a broad, thickish girdle of indifferent tissue 
closely connected with the outer tunic externally and in front, and 
with the generative blastema behind, in buds of -44th— jth of an 
inch long (figs. 21 and 22); but in larger zooids its tissue has under- 
gone a great change, and it has become a transparent mass, through 
which ramified corpuscles, like connective-tissue corpuscles, appear 
scattered (fig. 23). In this condition it is exactly analogous to the 
structure termed elzoblast in the Sa/pe by Krohn. Its bulk is now 
equal to a fifth or a sixth of that of the entire bud, but in subsequent 
stages (figs. 24, 25) it diminishes both absolutely and relatively in size 
and eventually it disappears. 
In buds th of an inch in diameter, the generative blastema 
remains in its primitive condition, except that it and the ovisac it 
contains, have increased in size. Its anterior pointed end is closely 
juxtaposed to the endostylic cone. In the zooid represented in fig. 23, 
which measured 5th of an inch in length, the generative blastema 
has become divided into two parts, the smaller of which remains in 
close apposition to the endostylic cone, while the larger, retaining its 
connexion with the posterior and upper wall of the mid-atrium, 
becomes widely separated from the other. The interval between the 
two is occupied by the elaoblast. Even before the separation has 
taken place, the larger portion has become distinctly differentiated 
into two parts, the ovisac, on the left, separating itself from a rounded 
mass of indifferent tissue, on the right. This last is the rudiment of 
the testis. From rounded, it becomes pyriform, the narrowed neck of 
the pear remaining in connexion with the atrial wall, and eventually 
