580 



BALDWIN SPENCER AND GEORGINA SWEET. 



We fail to see any relationship between the specially modi- 

 fied inner root-sheath of Monotremes and the appendicular 

 part of a feather. The sheath is formed perfectly indepen- 

 dently of the hair rudiment in the wall of the follicle, which, 

 with the development of the hair, becomes transformed into a 

 pit. A comparison of a transverse section of a developing 

 hair and feather will serve to show there is no real relationship 

 between the inner root-sheath of the one and the appendicular 

 part of the other. 



c /.rs 



Fig. E. Transverse section across a Monotreme hair lying in its follicle, at the 



level of the top of the dermic papilla. 

 Tig. F. — Transverse section across a developing feather (modified from Newton 



and Gadow).. to show the relative positions of the developing main 



shaft, after-shaft, and appendicular parts. 



d.p. Dermic papilla, m.k. Medulla of hair. coJi. Cortex of hair. 



c.h. Cuticle of hair. c.i.r.s. Cuticle of inner root-sheath. i.r.s. 



Inner root-sheath. o.r.s. Outer root-sheath, p.s. Primary shaft of 



feather. r. Rami. si.c. Layer continuous with stratum corneum 



( = cuticle of inner root-sheath of hair), m. Malpighian cells of follicle. 



U.S. After-shaft, d. Dermis. 



We have already stated that the bilateral symmetry in the 

 large hair of Ornithorhynchus is not a primary, but a secon- 

 dary feature, and the same remark holds true in the case of 

 the large size of the bulb and papilla. The early development 

 of these is precisely similar, so far as relative size is concerned, 

 to that of other mammals, and it is only with the secondary 

 modification of the hair to form in the one case a flattened 



