HAIRS OF MONOTREMES AND MARSUPIALS. 561 



while in transverse section they are seen to form more or less 

 roughly concentric bands, which in the lower part tightly 

 enclose the hair. If the corneous layer be traced downwards 

 towards the bulb it will be noticed that the corneous nature 

 gradually disappears, and that the network is really directly 

 continuous with the softer, undifferentiated, nucleated layers of 

 the inner part of the bulb, that is the part lying between the 

 outer root-sheath and the layer of columnar cells next to the 

 dermic papilla. On the other hand, at the opposite end of the 

 follicle the network is clearly continuous with the layer lyino- 

 immediately beneath the stratum corneum, which layers are in 

 their turn becoming gradually corneous. Owing to the way in 

 which the network closely envelops the hair which is growing 

 up through it, it is usually difficult at this stage to detect 

 exactly how far the hair has grown up. The central part of 

 the network, which lies in close contact with the hair, as well 

 as the tip of the latter, takes the stain very deeply, so that at 

 times this central part almost appears to be distinct from the 

 rest ; but anything like close examination at once shows that 

 it is only a special part of the general network which is most 

 deeply stained, but which is at the same time in direct con- 

 tinuation with the latter. 



It is at this stage that the lumen of the follicle really makes 

 its appearance, though as yet it is ill- defined, except at the 

 surface, though even here it has the form of a somewhat in- 

 definite tubular cavity crossed by a meshwork of cornified cells, 

 the substance of which is gradually breaking down to form a 

 cavity. So far as the follicle is concerned, the structure can 

 be best understood from the study of a series of transverse 

 sections. In figs. 18 to 20 we have represented three such 

 sections taken at different levels. Fig. 18 is taken through 

 the bulb in such a way that the tip of the papilla is just cut 

 through. The nuclei of the layers, which are, when traced 

 upwards, found to be continuous with the medulla and cortex 

 of the growing hair, are surrounded and almost hidden from 

 view by the pigment, which is now strongly developed. Out- 

 side these layers is a clearly marked series of nuclei, the layer 



