This very distinctive tissue--the muco-cartilage--is of very great importance in all questions of the origin of the skeletal tissues. In all descriptions of the skeletal tissues it has been practically disregarded until recent years when, besides my own observations, its distribution has been mapped out by Schaffer. Thus Parker, in his well-known description of the skeleton of the marsipobranch fishes, does not even mention its existence. Its importance is shown by its absolute disappearance at transformation and its non-occurrence in any of the higher vertebrates. It is entirely confined to the head-region, and its distribution there is most suggestive, for, as will be described fully later on, it forms a skeleton which both in structure and position resembles very closely the head-shields of cephalaspidian fishes. At the present part of my argument its more immediate interest lies in the method of tracing this tissue. For this purpose I made use of the micro-chemical reaction of thionin, a dye which, as shown by Hoyer, stains all mucin-containing substances a bright purple. Schaffer made use of a corresponding basophil stain, hæmalum. When stained with thionin, the matrix, or ground-substance of the branchial cartilages as well as the matrix or semi-fluid substance in which the fibrils of the muco-cartilaginous cells are embedded take on a deep purple colour, while the fibrous material of the cranial walls and other connective tissue strands, such as the perichondrium, are coloured light blue. Muco-cartilage, then, may be described as a peculiar form of connective tissue which differs from other connective tissue not only in its appearance but in {132}its chemical composition, for unlike white fibrous tissue it contains a large amount of mucin, and this tissue is the forerunner of the earliest cartilaginous vertebrate skeleton, the branchial bars of Ammocoetes. The conclusions to which we are led by the study of the structure, position, and mode of origin of these primitive cartilages of Ammocoetes may be thus summed up:-- 1. The immediate ancestor of the vertebrate must have possessed a peculiar fibrous tissue--the ground-substance of which stained deep purple with thionin--in which cartilage arose. 2. The cartilage so formed was not like hyaline cartilage, but resembled in a striking manner parenchymatous cartilage. 3. This cartilage was situated partly in two axial longitudinal bands, partly as transverse bars, which supported the branchial apparatus. THE PROSOMATIC OR BASI-CRANIAL SKELETON OF AMMOCOETES. Before searching for any evidence of a similar tissue in any invertebrate group, it is advisable to consider the other portion of the cartilaginous skeleton of Ammocoetes, which consists of the trabeculæ, parachordals and auditory capsules--the basi-cranial skeleton--and is composed of hard, not soft cartilage.