DEVELOPMENT OP AMPHIURA SQUAMATA. 135 



genital rachis) is ventral in the interradii^ but at the sides of 

 the radii it mounts to the dorsal surface, thus passing over the 

 genital bursse, which are involutions of the integument close to 

 each side of the arm. On the interradial side of these bursse 

 the rachis bears the ovary ; on the radial side it gives oflF a 

 branch terminating in the testis, which runs into the short 

 extension of the body-cavity into the arm, called by Russo 

 (11) the testicular chamber. 



Fig. 2 a is a longitudinal section of the disc passing through 

 the ovoid gland of an adult. Fig. 2 5 is another section from 

 the same series, and fig. 2 c is a horizontal section of the gland 

 of another individual. 



In the centre of the organ one sees the stone canal (st. c.) 

 with its characteristic columnar epithelium, which dorsally 

 passes into the flattened epithelium of the water-vascular ring 

 {w. V. r.). A section of the pore canal (p. c), leading to the 

 madreporie pore, is seen in the body-wall. I find three spaces 

 in connection with the ovoid gland, and, as they have all been 

 confused together under the name of axial sinus, I have called 

 them sinus «, b, and c respectively, though I do not thereby 

 mean to imply that they belong to the same category. 



Sinus c is the thin-walled ampulla, into which both stone 

 canal and pore canal open, and it has no connection what- 

 ever with any other cavity. Sinus a is the part of the 

 aboral sinus which conies in contact with the ovoid gland, and is 

 flattened out on its external surface. Sinus b is the axial sinus 

 proper, which is principally developed round the distal swollen 

 end of the gland, but also extends up on to its sides. These 

 three spaces — sinus a, sinus b, and sinus c — are all 

 quite independent of each other. I have traced them in 

 every well-preserved series which I have examined. As to the 

 structure of the gland itself, I find that it often presented the 

 areolated texture referred to above. This I believe to be 

 entirely due to imperfect preservation. When suffi- 

 ciently strong osmic acid is used, and precautions taken to 

 ensure its penetration (by opening the animal whilst living), 

 one sees a uniformly staining plasma (figs. 2 b, 2 c) supported 



