. The elasmobranch fishes . B Fig. 130. Developing thymus gland, Spinax. (From Fritsche.) A. Transverse section. B. Section of gland magnified. , anterior cardinal sinus; did., notochord; c?.^*, third and fourth clefts; ht., heart; my., myotome; , neutral tube; pli., pharynx; tlu, thymus gland. nodules may be present in Heptanchus. In Spinax, Acanthias, (s, and Scylliiini four of these, corresponding to the first four branchial clefts, are present, and, in the embryo of Spinax, transitional thymus buds have also been found over the spiracular and the fifth clefts. In the ray
. The elasmobranch fishes . B Fig. 130. Developing thymus gland, Spinax. (From Fritsche.) A. Transverse section. B. Section of gland magnified. , anterior cardinal sinus; did., notochord; c?.^*, third and fourth clefts; ht., heart; my., myotome; , neutral tube; pli., pharynx; tlu, thymus gland. nodules may be present in Heptanchus. In Spinax, Acanthias, (s, and Scylliiini four of these, corresponding to the first four branchial clefts, are present, and, in the embryo of Spinax, transitional thymus buds have also been found over the spiracular and the fifth clefts. In the rays thus far studied a similar number is present lying back of the spiracle and between the gill pockets and the lateral line. Figure 22d gives a dorsal view of the embryo of Urolophus in which the nodules of the thymus are seen between the gill pockets and the lateral line canal. In development the thymus arises as an anterodorsal thickening of the epithelium of the gill pouches (th., fig. 130a). These thickenings as we have said may represent the spiracular and the five branchial pockets in pentanchid Elasmobranchs, but the first and the last never pass the rudimentary stage. Figure 122 of Heptanchus cinereus shows that each thymus nodule has the form of a bunch of grapes. A highly magnified section through the thymus of Spinax (fig. 130b) shows two types of cells, one outer and larger, and the other a more deeply located, small, round cell. Among them are to be found occasional lymphocytes, and it has been questioned whether the smaller cells of the thymus are not trans-
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