7 
growth of the rostrum is limited to its distal part. One may look 
on this distal part as a growing tip—that there is isolated in 
the tip of the snout of a spoonbill larva a cell, or group of 
cells, which grow more rapidly than do those of the remainder of 
the animal; furthermore, that this cell, or group of cells, is 
carried forward by its own growth leaving behind it cell descen- 
dants which grow at the same rate as the rest of the body, i.e., 
more slowly. Since this growing tip apparently has the capacity 
to produce cartilage at a greater rate than its cell descendants 
in the same concentrations of cartilage-forming stuffs, it will 
tend to build up a surplus which cannot be maintained at an isogon- 
ic growth level once this growing tip is destroyed or is changed 
in its growth potentialities. As the gross amount of the cell 
descendants of this growing tip increases relative to the rest of 
the body, the blood supply reaching the tip must traverse more and 
more cartilaginous tissue and hence may become poorer and poorer 
in cartilage-forming substances. It seems reasonable that this 
starvation will reach a point where the growing tip is either 
destroyed altogether or is so altered that its cells no longer 
multiply more rapidly than those of other parts of the rostrum. 
As one inspects the outstanding anatomical features of 
Folyodon in the material which we have available, it is apparent 
that the rostrum and the opercular flaps are the last organs to 
be differentiated. All other organs and major parts already have 
made their appearance and have passed through the initial stages 
of their growth history. We should look upon the heterogonic 
growth of the rostrum and the operculum as the normal growth beha- 
vior of parts which have been delayed in their differentiation 
while other large organs of. the body have already been differentiat- 
ed and have come into growth equilibrium with each other. 
The rostrum is in a variable and unregulated condition, 
It may be looked upon as an organ which has been recently evolved 
and not yet coordinated with the rest of the body, or else, while 
it may be quite old, it has not been subjected to rigorous selec- 
tive action. As one compares a series of specimens of Folyodon 
of the same body length one finds that such anatomical features ' 
as body contour, length, width, and insertion of fins, size of 
eye, and expanse of mouth are very uniform and fixed in their 
proportions. In contrast with the fixity of these morphological 
features the length of rostrum varies widely. It is apparent, 
however, that individuals with short rostra show a compensating 
increase in width. The lengths of opercular flaps not only vary 
from fish to fish but are usually different on the right and left 
sides. The rostrum of the spoonbill, as compared with the snouts 
of other fishes, may be considered as an organ which has undergone 
great changes since the ancestors of Folyodon split off from other 
fishes, and is twice as large as that of the Eocene spoonbill, 
Crossopholis. 
