THE FEATHERS 151 



The pineal body develops precociously in the penguin. It is a short process of the 

 roof of the thalamencephalon as early as stage 25, with a ring of nervous tissue surround- 

 ing it which forms the habenular ganglion. The appearance of the brain as a whole at this 

 stage (Fig. 5 a) recalls that of the poorly developed brain of the lamprey. The hemi- 

 spheres and the optic lobes in both cases are almost of equal size, and the pineal bodies 

 on the dorsal surface of the two brains strikingly resemble one another. 



In the stage figured in Fig. 5 b, the glandular tip of the pineal body is shown free from 

 the surface of the brain. It has developed a large peduncle and its size is temporarily 

 disproportionate to the rest of the brain. In this way it anticipates the subsequent 

 growth of the hemispheres and cerebellum (Fig. 5 c) and retains its position on the 

 dorsal surface of the fully developed brain. In its later stages the organ undergoes con- 

 siderable degeneration and does not at any time show traces of eye structure. 



THE FEATHERS 



Feather papillae first show themselves in the skin of the tail (stage 34, Plate III, 

 fig. 2, rt). According to Anthony and Gain (1915, p. 22) the later papillae arise in 

 isolated feather tracks which spread and fuse with one another as development proceeds. 

 It is a remarkable fact that the last feather rudiments to appear are those which occur on 



the wings. 



In actual distribution the feathers of penguins are unique. They occur uniformly all 

 over the body in the aduU and only one apterium persists (Pycraft, 1907, p. 2). In the 

 embryo, naturally, this exceptionally complete coat of feathers is preceded by an equally 

 complete covering of feather papillae (cf. Plate IV, fig. i). But between the definitive 

 feathers and the papillae there intervene no fewer than two generations of nestling 

 down (Pycraft, 1907, p. 11). The embryo hatches with the first of these, and with their 

 development from the papillae the following observations are alone concerned. 



A fragment of skin bearing feather papillae and filaments was dissected from the wing 

 of the youngest Ring penguin embryo. It will be remembered that this embryo was at 

 approximately the same stage of development as the Gentoo embryo figured on Plate IV, 

 fig. I. The piece of skin was drawn (Fig. 6 a) and sections were then cut longitudinally 

 and transversely through the papillae and also through the filaments. The sections 

 through the papillae (Fig. 6 b, c) show the thickened epidermis overlying a dermal layer 

 which is condensed in the region of each individual papilla to such a degree that it has 

 an almost granular appearance. In the epidermis Wohlauer (1902) made out three 

 different types of cell: (i) a surface layer of flattened cells forming the "epitrichium", 

 (ii) a layer of intermediate cells, and at the base (iii) a layer of cylindrical cells which may 

 or may not be double. The same sequence of epidermal layers is characteristic of the 

 early stages in the development of the normal reptilian scale (cf. Kerr, 19 19, p. 71), and 

 the view that feathers can be derived from conical papillae of this type is therefore well 

 founded (Boas, 193 1, p. 563). There are strong arguments, however, against the opinion 

 that the feathers are in fact to be regarded as modified scales. These are set out else- 

 where (e.g. Pycraft, 1907, p. 3 ; Ewart, 1921, p. 634, etc.), and the controversy remains 



