1906.] ANATOMY OF CENTROPHORUS CALCEUS. 883 



inches of the cloaca, when the walls become thickened to form an 

 extruding organ. 



Remarks on the Anatomy of the Embryo. 



It was my original intention to include in the present paper a 

 complete anatomical account of the embryo (of the stage of growth 

 mentioned above) of G. calceus as well as of the adult, but circum- 

 stances at present rendering this impossible to carry out, I must 

 content myself with the following brief remai'ks. 



Five embryos were found in the uterine portions of the oviducts 

 of my specimen of C. calceus — four on one side and one on the 

 other, if I remember aright — all about five centimetres in length 

 (PI. LXII. fig. 20). They possessed a large yolk-sac attached 

 between the pectoral fins ; external filamentous gills protruded 

 from the five pairs of gill-clefts, and to a smaller extent from the 

 spii-acle (in front of which was a depression in the skin which 

 resembled a cleft) ; the eyes were large and hemispherical, and 

 the three primary brain- vescicles were as usual distinctly visible 

 from the dorsal surface. One noteworthy peculiarity is that 

 the narial aperture on each side is subdivided into two — a 

 peculiarity not found elsewhere, so far as I know, in Elasmo- 

 branchs. The snout is not elongated in the embryo as in the 

 adult and the mouth is but slightly curved ; also the caudal fin 

 extends forwards ventrally to the anterior level of the posterior 

 dorsal fin, perhaps so repi-esenting a i-udimentary anal fin Avhich 

 of course is absent in the adult. 



On opening the body-cavity of one of the embryos, I found the 

 liver and alimentary tract to have the conformation depicted in 

 fig. 21 (PL LXII.). The liver-lobes are long and tapering, as 

 in the adult. The stomach of the embryo narrows as it approaches 

 the commencement of the spiral-valve intestine, and joins it in 

 the manner indicated. I have not yet studied my sections through 

 this region of the embryonic gut. The bile and umbilical ducts 

 enter the large intestine as shown. The rectal gland and cloacal 

 aperture are well-marked. 



Dorsal to the gut, and extending from the cloaca to near the 

 anterior extremity of the body-cavity, lie the two rows of meta- 

 merically-arranged kidney-tubules, beautifully distinct in the 

 actual specimen, but which I have not been able to show very 

 well in fig. 22, their position alone being indicated. 



The tubules open into the kidney-ducts, which extend more 

 anteriorly, suddenly diverging from the median line as they do so. 

 Situated internally to the rows of kidney-tubes are the two thin 

 genital ridges, extending from the level of the anterior kidney- 

 tubules half-way to the cloaca. 



Sections through the posterior caudal i-egion of the embryo and 

 adult distinctly show a blood-vessel of considerable size lying at 

 the base of the neural spine of the vertebra, i. e. just above the 

 nerve-cord (text-fig. 138). This vessel, which in the sections con- 

 tains numerous blood-corpuscles, is apparently a vein. Towards 



