AKT. 2G GUMS OF A PORPOISE MILLER 3 



at base but that each is capped by a conspicuous hiyer of greatly 

 thickened epidermis. At the base of each elevation may be seen 

 a thickened mass of corium from which arises a group of papilla? 

 (about 0.125 mm. in width) extending upward through the first 

 layer of the epidermis. This layer (about 1.05 mm. in thickness), 

 whose cells contain nuclei which tend to be highly refractive, ap- 

 pears to be the sti'atwn gei^Tninativum. It is followed by a second 

 layer (also about 1 mm. in thickness) in which the nuclei are dark, 

 seemingly the stratum granulosum. At the peripheral surface it 

 is elevated into the papillae which cover the entire surface of the 

 gums. Each superficial papilla is subtended by a papilla of the 

 corium; and in sections which have been cut at appropriate levels 

 the stratum gmnulosn/ni can be seen to be traversed by a narrow 

 line of crowded nuclei extending from the tip of the corial papilla 

 outward into the substance of the terminal papilla. In some prepa- 

 rations the surface of the terminal papillse is covered by a thin layer 

 of flattened cells which may represent the stratuvi corneum, but 

 which has, in most instances, apparently been stripped away in 

 course of preparation. The superficial appearance of this outermost 

 laj^er is shown in the upper left hand portion of Figure a on Plate 4. 

 Here it ma}'^ be seen to cover most of the underlying papillae com- 

 pletely. Occasionally it is pierced by a minute aperture over the 

 tip of a papilla, and occasionally an entire papilla or a small group 

 is entirely laid bare. 



The general conditions which I have just described, both macro- 

 scopic and microscopic, seem to be not essentially different from those 

 which Tullberg figured as occurring at early stages of the develo])- 

 ment of the baleen plates in Sibhaldus.* Comparing TuUberg's 

 Plate 4 with my Plate 4 the general resemblance in gross structure is 

 at once evident, allowance being made for the much more highly 

 specialized condition present in Sibhaldus. The true teeth in the 

 young baleen whaje have disappeared. The gum teeth are com- 

 pressed along the axis of the jaw; their bases have spread inward 

 to occupy a much greater area of the palate; they have increased 

 in heiglit. The papillae on the palate have become elongated and 

 filamentous. Otherwise there is no important change. The micro- 

 scopic structure in Phoccenoides could be fairly well represented by 

 TuUberg's Plate 5, Figure 23, if the drawing were reduced to about 

 one-third of its present height (the width remaining unaltered), the 

 papilla) were represented as single instead of in pairs, the terminal 

 papilliie were cut off at the level of the lower letter c, and a dividing 

 line between the stratum germinativum and stratum- granidosum were 

 indicated. 



♦Nova Ac(a Reg. Soc. Sci. Upsal., ser. 3, vol. 11, pi. 4, figs. 19-20, and pi. 5. fig. 23. 

 1883. 



