550 BALDWIN SPENOER AND GEOliGINA SWEET. 



above the surface, aud we are agaiu indebted to Dr. Gregg 

 Wilson for pieces of skin of an embryo of the same age. Owing 

 to the fact that in different parts of the body the hairs are not all 

 developed to the same extent^ we have been able to study various 

 stages — in the youngest one of which the follicle is only just 

 formed, while in the oldest the largest hairs are well developed. 



We are indebted to Professor J. T. Wilson and Mr. J. P. 

 Hill for the opportunity of consulting literature which was 

 unavailable to us in Melbourne. 



Whilst as the result of a considerably greater supply of 

 material, which is also probably in a better state of preservation 

 for histological work than was that available to Mr. Poulton, 

 we have been led to conclusions different from those to which 

 he came, especially in regard to the question of the develop- 

 ment of the hair in an open and not a closed tube — conclusions 

 to which, moreover, we feel sure that Mr. Poulton would have 

 come had he been able to study earlier stages — we desire to 

 express our indebtedness to the assistance which we have 

 received from his work, especially combined as it was with a 

 valuable resume of previous work compiled by Dr. Benham, 

 certain of the memoirs dealt with by the latter being unavail- 

 able to us. We are at the present time engaged upon an 

 investigation of the development of hairs in Marsupials, but 

 have thought it better to divide our work into two parts, and 

 the following deals simply with the structure and development 

 of hairs in Monotremes. 



To Professor G. B. Howes we are much indebted for his 

 kindness on this, as on other occasions, in revising the proof. 



General Structure and Arrangement of the Hairs. 

 The presence of large and small hairs aud their arrangement 



has been dealt with by various authors, Leydig being the first 



to point out that the small hairs are arranged in bundles with 



a common follicular neck. 



Leydigi described in Ornithorhynchus the presence of four 

 1 " Ueber die ausseren bedeckungen der Saugetliiere," ' Arch, fiir Anat. 



Phys. und Wiss. Med.,' 1859, pp. 677— 7i7. 



