DEVELOPMENT OF AMPHIURA SQUAMATA. 141 



swelling of this to form a blood-vessel^ and it can only be 

 traced in the few sections of the nerve-ring which also contain 

 the ovoid gland. 



As to radial blood-vessels — in many sections of the radial 

 nerve-cord — there does appear at first sight to be a small 

 round structure, in the mid-dorsal line, between two masses of 

 ganglion-cells. But if one uses careful double staining and 

 examines successive sections, one sees that it is often absent — 

 as, for instance, in fig. 21. There the ganglion-cells with nuclei 

 and plasma cover the whole dorsal surface of the cord. 

 Further, when one looks closely at the sections where there is 

 an appearance of a blood-vessel (cf. fig. 22) one sees that the 

 nuclei of its wall are exactly like the ganglion nuclei ; that its 

 clot stains almost if not quite the same as the plasma of the sur- 

 rounding cells. It is limited by no cell layer from 

 the nerve-fibres below, and its boundary towards 

 them is often jagged and uneven, and the angles of 

 this outline run out into vertical fibres, so as to leave 

 no doubt in my mind that the so-called blood-vessel is merely 

 composed of the cell plasma of two or three rather larger dorsal 

 ganglion-cells which are prolonged into these vertical fibres. 

 A remarkable confirmation of this view is obtained by the 

 study of specimens which have remained too long in osmic 

 acid {vi. d., fig. 23). Here the mass of nerve-fibres is strongly 

 shrunk and its shape altered, so that its dorsal outline is far 

 less convex. At the sides it is more or less wrenched away from 

 the dorsal ganglion-cells, leaving an artificial space; but in the 

 middle one or two adhere, and produce the appearance of a 

 blood-vessel. As to the so-called branches of the haemal 

 system which go to the alimentary canal, these seem to me to 

 be nothing more than mesenteric bridles ; one very marked one 

 is inserted just at the base of the ovoid gland, and in some 

 young specimens there appears to be a prolongation of the 

 peculiar tissue of the gland along it. In adults its relations 

 are obscured by the great extension of the genital bursse. 



