Any trace of a heart such as we have just described must be sought for in Ammocoetes between the central nervous system and the mid-line dorsally. Now, in this very position a large striking mass of tissue is found, represented in section in Fig. 73, _f_. It forms a column of similar tissue along the whole mid-dorsal region, except at the two extremities; it tapers away in the caudal region, and headwards grows thinner and thinner, so that no trace of it is seen anterior to the commencement of the branchial region. It resembles in its dorsal position, in its shape, and in its size a dorsal heart-tube such as is seen in Limulus and elsewhere, but it differs from such a tube in its extension headwards. The heart-tube of Limulus ceases at the anterior end of the mesosomatic region, this fat-column of Ammocoetes at the posterior end. In its structure there is not the slightest sign of anything of the nature of a heart; it is a solid mass of closely compacted cells, and the cells are all very full of fat, staining intensely black with osmic acid. Nowhere else in the whole body of Ammocoetes is such a column of fat to be found. It is not skeletogenous tissue with cells of the nature of cartilage-cells, as Gegenbaur thought and as Balfour has depicted (Vol. II., Fig. 315) in his 'Comparative Embryology,' as though this tissue were a part of the vertebral column, but is simply fat-cells, such as might easily have taken the place of some other previously existing organ. {182}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 _ex 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 Ammocoetes, is not found in the higher vertebrates, so that it possesses, at all events, the significance of being a peculiarity of ancient times before the vertebrate skeletal column was formed. I mention it here in connection with my view as to the origin of vertebrates, because there it is, 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. [Illustration: FIG. 73.--SECTION THROUGH THE NOTOCHORD (_NC._), THE SPINAL CANAL AND THE FAT-COLUMN (_F._), OF AMMOCOETES, DRAWN FROM AN OSMIC PREPARATION.