508 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
diphyodont state, tlie more anterior teeth being always most affected. 
(3) In the lower Mammals the first set is still dominant ; in the higher, 
the second. (4) The number of sets has been reduced in proportion as 
individual teeth have been specialised. 
He rejects Leche’s idea that the replacement-set is a new acquisition, 
notes Schwalbe’s corroboration of the theory of fusion, and criticises 
Hoffmann for combating this without sufficient consideration. 
Development of Teeth in Ruminants.* — Herr A. Hoffmann has 
studied the development of the sheep’s teeth, with particular reference 
to the crown-cement. Some of his results are the following: — From 
palaeontological as well as embryological evidence, it is plain that the 
fourth tooth on the lower jaw is a canine which has become like an 
incisor. In the region of these teeth, the lower jaw is separated from 
the lower lip by a deep groove, the lip-groove. The same is to be seen 
above and below in the region of the back teeth, but on the upper jaw 
in front, where teeth are absent, the groove though present is less dis- 
tinctly developed. The lip-groove is suppressed in proportion to the 
suppression of teeth. 
The development of the lower front teeth occurs from in front back- 
wards ; the dental ridge is continued uniformly over the region of the 
most anterior premolar, which does not develop; in the upper dental 
ridge there are no rudiments of the lost incisors ; the replacement-ridge 
in the region of the permanent premolars, is differentiated from behind 
forwards, and the same is true of the rudiments of all the back teeth. 
Of the insunk epithelium only the high cylindrical cells of the 
stratum mucosum which lie directly on the papilla can be regarded as 
equivalent to the enamel organ of lower Vertebrates. It is developed 
exclusively from the lingual fold of the dental ridge. The formation of 
the crown-cement in Ruminants occurs without the intervention of a 
cartilage-stage, by the direct ossification of the layer of the tooth-sac 
overlying the enamel surface. The main condition of its development 
is that the production of enamel come to an end a sufficiently long 
period before the emergence of the teeth, and in Ruminants there is 
a characteristic hastening of degeneration in the enamel organ and a 
characteristically rich vascularity in the tooth-sac. 
The molars are ontogenetically equivalent to teeth of the first set. 
In all cases they arise from a perfectly simple single papilla, which 
secondarily becomes complicated. There is no fusion of the rudiments 
of several series. Of reduction first affecting the milk-series there is no 
evidence. 
Reduction first affects the second series, and this may lead to a 
stronger development of the corresponding teeth in the first series. 
Hence the strength, persistence, and differentiation of those teeth, such 
as the molars of Placentals, which have no successors. 
Experimental Embryology.! — Prof. 0. Hertwig has made further 
experiments with the ova of Bana esculenta and B. fuscct , subjecting 
them to the influence of salt solutions. The addition of a little salt 
(•l-*6 per cent.) results in the production of what may be called 
* Zeitschr. f. wiss. Zool., lviii. (1894) pp. 566-617 (1 pi.), 
t Arch. f. Mikr. Anat., xliv. (1895) pp* 285-344 (3 pis.). 
