578 BALDWIN SPENCER AND GEORGINA SWEET. 



Oruithorhynchus or Echidua develops in what can be correctly 

 called an open tube. In the earlier stages — indeed, until the 

 hair is well formed and has reached a considerable distance 

 up the follicle — development takes place in a solid follicle. 

 Possibly the latter may open to the surface at a slightly earlier 

 stage than it does in some other mammals, but the one im- 

 portant point in this connection is that the hair-follicle has the 

 form of a solid downgrowth, and that the early stages during 

 which the hair is laid down are absolutely indistinguishable 

 from those of other mammals. Figures which represent the 

 condition in one of the earliest stages of the follicle are pre- 

 cisely similar to those of corresponding stages in other mam- 

 mals, and, with the elongate nuclei at the base of the follicle, 

 correspond so closely to as to be in fact indistinguishable from 

 those of Marsupials, such as Macropus and Perameles. The 

 resemblance between the two is so complete, that we think 

 there can be very little doubt but that in Monotremes, just as 

 in Marsupials, in which our observations (to be published later) 

 to a large extent confirm those of Maurer, the very earliest in- 

 dication of the hair will be found to take the form of an elonga- 

 tion of the lowest epidermic cells. At all events, the figures now 

 given show (1) that the hair cannot be described as developing 

 in an open tube ; and (2) that the earliest trace is not formed 

 on the surface, and then sinks with the deepening tube. 



The next point which we desire to lay emphasis upon is that 

 in these lowest mammals, and indeed in all mammals, the hair 

 is a radially symmetrical structure ; by which we mean to imply 

 not that hairs may not possibly be derived from structures 

 which originally possessed a bilateral symmetry, but that, 

 from the earliest moment at wliich we get the first rudiment 

 of the hair itself laid down, the structure takes on a radial 

 symmetry, and that any bilateral symmetry, such as may be 

 found in the well-developed hairs of both Oruithorhynchus and 

 Echidna, is a secondary and not a primary feature.^ 



1 As we shall show later, the flattened bristles of such a Marsupial as 

 Perameles are, iu their early development, radially symmetrical, the bilateral 

 symmetry, just as iu Oruithorhynchus, being a secondary feature. 



