ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
543 
their relation to their successors is more typical ; in the rabbit where 
the tooth of the second dentition is more largely developed, the milk 
predecessor has partially lost its independent enamel organ ; and lastly 
in the mouse, where the successional tooth is still more out of propor- 
tion, the enamel organ of the milk tooth has completely disappeared, the 
tooth itself being very variable and when present excessively minute.” 
In the rabbit, about whose dentition confusion still exists, the first 
milk incisors above and below never cut the gum, but are absorbed in 
utero ; the second milk incisors (none below) are functional, but are shed 
g 
at about the third week of extra-uterine life ; the milk premolars - are 
functional, but are shed at about three weeks. 
From his study of the molars, Mr. Woodward is led to suggest that 
“ the functional molars of the Mammalia are to be assigned to the same 
set as the replacing teeth of the anterior part of the jaw, i. e. the second 
dentition, unless we believe in a pre-milk dentition, when we must 
term this set the third dentition.” 
Homologies and Origin of Mammalian Hair.* — Prof. E. B. Poulton, 
taking as his text the structure of the bill and hairs of Ornitliorhynchus 
paradoxus, offers some criticisms of earlier views and some general con- 
clusions of his own. He is inclined to oppose Gegenbaur’s and Kolliker’s 
distinction between hairs on the one hand, and feathers and scales on 
the other, which is founded on the fact that the former are developed 
from the base of a solid epithelial downgrowth, and the latter from an 
epithelial upgrowth. In OrnithorJiynchus an open tube replaces the 
solid downgrowth of the higher Mammalia, and Prof. Poulton thinks 
that this is a clear demonstration that the solid cylinder is the abbre- 
viated representative of the open tube, and that the significance of 
Gegenbaur’s distinction between feather and hair falls to the ground. 
Another important point in OrnithorJiynchus is the great length of the 
papilla projecting through the bulb into the lower part of the hair, for 
this suggests a previous development like that of a scale or feather from 
the surface of the epidermic covering of a papillary core traversing the 
structure from base to apex. Further confirmation is afforded by the 
axial rod of soft protoplasmic cells forming the medulla of hair ; for, it 
is suggested, a shortening papillary core, surrounded by cells of the 
rete mucosum superficially undergoing cornification, would tend to leave 
just such an indication of its former presence. 
Some of the features to which Maurer points as characteristic of 
feathers are equally characteristic of the hairs of Ornithorhynchus ; in 
obliquity of direction and in distinction between an upper and a lower 
surface, the large hairs of that Monotreme resemble feathers. 
As the new large hair of Ornithorhynchus is far advanced in develop- 
ment before the old one is shed we have practically a periodical shed- 
ding of the stratum corneum (hair) preceded by the formation of a new 
stratum corneum (new hair) below. “ The succession of hairs is, in fact, 
the one exception to the gradual wearing off of the superficial corneal 
cells in Mammals which is so important a difference between them and 
Reptiles.” 
* Quart. Journ. Micr. Sci., xxxvi. (1894) pp. 143-99 (3 pis.). 
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