HAIRS OF MONOTREMES AND MARSUPIALS. 549 



The Structure and Development of the Hairs of 

 Monotremes and Marsupials. 



Part I. — Monotremes. 



By 



Baldwin Spencer, M.A., 



Professor of Biology iu the Uuiversity of Melbourne, 



and 



Oeoi'g:ina Sweet, M.Sc., 



University of Melbourne. 



With Plates 44—46. 



In his paper dealing with the structure of the bill and 

 hairs of Ornithorhynchus/ Mr, Poulton has described the 

 structure of the hair in an embryo of Ornithorhynchus 

 measuring 8*3 cm. long, in which the larger hairs had appeared 

 above the surface of the skin. Thanks to the kindness of 

 Professor J. T. Wilson, of Sydney, we have been able to study 

 much earlier stages as developed in an embryo measuring 40 

 mm. in length, the same embryo upon which he and Dr. Martin 

 have already worked when studying the structure of the bill. 

 To Mr. Dudley le Souef we are indebted for an embryo 

 measuring IK cm., iu which the hairs have not yet appeared 

 above the surface, and to Dr. Gregg Wilson for pieces of skin 

 of an embryo of the same age. In the case of Echidna we 

 have been able to study the structure of an embryo measuring 

 55 mm. in length in which the tips of the larger hairs, which 

 subsequently become modified into spines, have just appeared 

 ' ' Quart. Jouru. Micr. Sci.,' vol. xxxvi, pt. 2, p. 143. 



