182 THE ORIGIN OF VERTEBRATES 
I do not know how to decide the question which thus arises. 
Supposing, for the sake of argument, that this column of fat-cells 
has really taken the place of the original dorsal heart, what criterion 
would there be as to this? The heart cx hypothesi having ceased to 
function, the muscular tissue 
would not remain, and the 
space would be filled up, 
presumably with some form 
of connective tissue. As 
likely as not, the connective 
tissue might take the form 
of fatty tissue, the storage 
of fat being a physiological 
necessity to an animal, while 
at the same time no special 
organ has been developed 
for such a purpose, but fat 
is being laid down in all 
manner of places in the 
body. 
This dorsal fat-column, 
as it is seen in Ammoccetes, 
is not found in the higher 
vertebrates, so that it pos- 
sesses, at all events, the 
significance of being a pecu- 
liarity of ancient times 
before the vertebrate skele- 
Fig. 78.—SECTION THROUGH THE Norocnorp tal column was formed. 
(nc.), THE SprnaL CaNAL AND THE Far- I mention it here in 
COLUMN (f.), OF AMMOC@TES, DRAWN FROM tt ith : 
AN Osmic PREPARATION. connection with my view 
sp. ¢., spinal cord; gl., glandular tissue filling 8 to the origin of verte- 
the spinal canal; sk., Gegenbaur’s skeleto- brates, because there it is, 
Beton cells Wg PR, in the very place where the 
dorsal heart ought to have been. For my own part, I should not have 
expected that a muscular organ such as the heart would leave any 
trace of itself if it disappeared, so that its absence in the dorsal region 
of the vertebrate does not seem to me in the slightest degree to 
invalidate my theory. 
