Dohrn[25] was the first to bring into prominence the degenerate character of the Cyclostomata. I cannot however assent to his view that they are descended from a relatively highly-organized type of Fish. It appears to me almost certain that they belong to a group of fishes in which a true skeleton of branchial bars had not become developed, the branchial skeleton they possess being simply an extra-branchial system; while I see no reason to suppose that a true branchial skeleton has disappeared. If the primitive Cyclostomata had not true branchial bars, they could not have had jaws, because jaws are essentially developed from the mandibular branchial bar. These considerations, which are supported by numerous other features of their anatomy, such as the character of the axial skeleton, the straightness of the intestinal tube, the presence of a subintestinal vein etc., all tend to prove that these fishes are remnants of a primitive and prægnathostomatous group. The few surviving members of the group have probably owed their preservation to their parasitic or semiparasitic habits, while the group as a whole probably disappeared on the appearance of gnathostomatous Vertebrata. [25] _Der Ursprung d. Wirbelthiere_, etc. Leipzig, 1875. The ripe ovum of Petromyzon Planeri is a slightly oval body of about 1 mm. in diameter. It is mainly formed of an opaque nearly white yolk, invested by a membrane composed of an inner perforated layer, and an outer structureless layer. There appears to be a pore perforating the inner layer at the formative pole, which may be called a micropyle (Kupffer and Benecke, No. 79). Enclosing the egg-membranes there is present a mucous envelope, which causes the egg, when laid, to adhere to stones or other objects. [FIG. 37. LONGITUDINAL VERTICAL SECTION THROUGH AN EMBRYO OF PETROMYZON PLANERI OF 136 HOURS. _me._ mesoblast; _yk._ yolk-cells; _al._ alimentary tract; _bl._ blastopore; _s.c._ segmentation cavity.] Impregnation is effected by the male attaching itself by its suctorial mouth to the female. The attached couple then shake together; and, as they do so, they respectively emit from their abdominal pores ova and spermatozoa which pass into a hole previously made[26]. [26] Artificial impregnation may be effected without difficulty by squeezing out into the same vessel the ova and spermatozoa of a ripe female and male. The fertilized eggs are easily reared. Petromyzon Planeri breeds during the second half of April.