556 BALDWIN SPENCER AND GEORGINA SWEET. 



which gives any colour to the idea that the earliest trace of the 

 hair is formed on the surface and subsequently sinks. In fig. 

 9, the nuclei of the outer layer of the follicle, which are 

 continuous with those of the Malpighian layer, are seen to be 

 somewhat elongated, and the central part of the follicle is 

 occupied by a mass derived from, and continuous with, the 

 middle part of the epidermis, and in which the nuclei are 

 evidently undergoing rapid proliferation. The stratum corneum 

 extends continuously over the external surface, and there is 

 not the faintest trace of a tubular downgrowth ; the follicle at 

 this very early stage is, in fact, precisely similar to that of all 

 other mammals. 



Stage 2. — The difference between this and the preceding 

 stage consists in a slightly greater elongation of the follicle 

 and in an aggregation of the dermic cells at the base of the 

 follicle affording the earliest indication of the formation of the 

 future dermic papilla. Not infrequently, as shown in fig. 10, 

 there is a trace of a blood-vessel running up towards the base 

 of the follicle. There is still no indication of anything like a 

 tube, the stratum corneum running continuously across above 

 the follicle. It will be noticed that in these early stages there 

 is no outline of cells to be distinguished except to a certain 

 extent in the stratum corneum ; elsewhere, and until later 

 stages, there are simply series of nuclei, those of the Malpighian 

 layer being distinguished as usual by their more elongate form 

 and definite arrangement. 



Stage 3 (figs. 11, 12). — Poulton was, we believe, the first 

 to draw attention to the fact that in regard to the arrange- 

 ment of the hairs in Ornithorhyiichus there is a distinct 

 bilateral symmetry ; this he also showed to obtain in respect 

 to the structure of the large hair. Whilst undoubtedly the 

 latter statement is true if the hair be examined at a relatively 

 late stage of development, — a stage such, for example, as 

 Poulton's figures refer to, — we think, as will be shown later on, 

 that this bilateral symmetry is only of a secondary nature and 

 is not of any importance from a phylogenetic point of view. 

 There is, however, at this early stage an indication of an original 



