582 BALDWIN SPENCER AND GEOKGINA SWEET. 



It must also be remembered tlmt iu the case of any comparison 

 between the two we must compare not the hair and the highly 

 developed quill or contour feather^ but the former and the 

 primitive nestling feather^ or " Neossoptile/' with its short 

 calamus and terminal bunch of " spikes.'^ In development 

 even this, the earliest form of feather, shows traces of bilateral 

 symmetry, while in the case of the hair we have a strongly 

 marked radial symmetry ; in the former the primitive symmetry 

 of a possibly simple ancestral scale is retained, while in the 

 latter it is lost before the rudiment of the hair is actually laid 

 down. It must also be remembered that we have no evidence 

 whatever as yet of any development of hair on the external 

 surface of the epidermis. From the lowest to the highest 

 mammals the development of the hair is practically identical, 

 the Monotremes even showing us nothing which can be re- 

 garded as primitive in the development of the actual hair. It 

 may, however, be worth while to draw attention to the fact that 

 in the Monotremes, and probably also in certain of the Marsu- 

 pials, the original follicle may give rise not only to the large 

 hair, but to small hairs which are developed in follicles budded 

 off from the large one, and that thus we may, perhaps, have 

 a homoplastic relationship between, on the one hand, the axial 

 part of a feather and the large hair, and, on the other hand, 

 between the appendicular part of a feather and the small hairs. 

 It appears to us that in the feather there is no representative 

 of the inner root-sheath in the hair which is to be regarded as 

 a special structure intimately associated with the growth in a 

 follicle, while in the hair there is no representative of the 

 feather sheath which is shed, and which is, it seems to us, 

 correctly compared by Maurer to the moulted cuticle of rep- 

 tiles. 



We shall return subsequently, when dealing with the deve- 

 lopment of hairs in the Marsupialia, to the question of the re- 

 lationship of the hair to other epidermic structures ; meanwhile 

 our conclusions with regard to the development of hair in 

 Monotremes may be summarised as follows : 



(1) The early developmeut of the follicle is precisely similar 



