132 



THE ELASMOBRANCH FISHES 



crease in size posteriorly. The lining of the pharynx is a continuation of that 

 of the buccal cavity, being made up of a deeper layer of comiective tissue and 

 a superficial epithelium; in the latter are located the mucous cells. Special 

 interest attaches to the pharyngeal region because of two structures associated 

 with it, the thymus and the thyroid glands. 



The thymus gland is an embryonic structure appearing as a series of nodules 

 connected into a chain above the gill pockets. We have seen that six such 





B 



Fig. 130. Developing thymus gland, Spinax. (From Fritsche.) A. Transverse section. 

 B. Section of gland magnified. 



a.G.s., anterior cardinal sinus; did., notochord; c?.^*, third and fourth clefts; ht., heart; 

 my., myotome; n.t., neutral tube; pli., pharynx; tlu, thymus gland. 



nodules may be present in Heptanchus. In Spinax, Acanthias, 3Iu.sfeli(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- 



