HAIRS OF MONOTREMES AND MARSUPIALS. 557 



bilateral symmetry, which in its turn soon becomes lost and 

 has no influence upon the form of the hair; in fact, this 

 bilateral symmetry completely disappears before there is any 

 trace of the hair proper. To show the close agreement of 

 Ornithorhynchus and Echidna in this respect we have drawn 

 sections which pass longitudinally through the follicle of a 

 future large hair in each animal at this stage. The earliest 

 indication of any modification of the end of the follicle to give 

 rise to a bulb, takes the form of a flattening from above down- 

 wards of the end of the follicle, attended by a slight pitting in 

 of the plate-like structure thus formed. On this plate 

 (separated from it by a slight space, evidently an artefact in 

 the section) lies a mass of dermic cells, the early rudiment of 

 the dermic papilla. In both cases, and especially in that of 

 Ornithorhynchus, the nuclei of the plate are especially 

 elongated. Tlie points a and h indicate, as seen in section, the 

 future lowest parts or rim of the bulb, the point a gradually 

 growing downwards ; while at the same time the dermic papilla 

 becomes more firmly established, the bulb becoming, as it 

 were, moulded upon the papilla. The follicle is still quite 

 solid, and there are indications that the nuclei in the central 

 part are beginning to arrange themselves so that their long 

 axes are parallel to the length of the follicle. This is espe- 

 . cially seen in the case of the section of Platypus (fig. 11). It 

 will be seen that in each follicle we can distinguish four parts : 



(1) that which lies nearest to the epidermis, and in connection 

 with which there can be seen in the section of Platypus a 

 small swelling, the earliest indication of the sebaceous gland ; 



(2) a slightly swollen part which indicates the position in which 

 the hair when developed is most tightly enveloped by the wall 

 of the follicle, and where, as will be shown subsequently, there 

 is a special modification in the inner root-sheath ; (3) a some- 

 what narrower part, which, as the hair grows, increases greatly 

 in length in proportion to the other parts ; and (4) the rudiment 

 of the bulb. 



Stage 4 (fig. 13). — The section figured is taken from an 

 Echidna, and shows the definite form of the papilla round which 



