ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
267 
(b) the permanent teeth (except the third premolar) and the deciduous 
third premolar, a set which he regards as homologous with the milk set 
in other mammals ; and (c) the third premolar and certain “ enamel 
germs ” lingual to the permanent teeth, — a “ progressive ” set which he 
regards as homologous with the permanent set in other mammals. 
According to Wilson and Hill, however, the persisting teeth of mar- 
supials are homologous with the second set mother mammals; the third 
deciduous premolar is a true milk-tooth, the prelacteal germs are de- 
generate milk-teeth, and the “ enamel germs ” are merely residues of the 
enamel ridge. 
Carlsson comes to the following conclusions : — 
(1) The deciduous third premolar belongs to the same generation as 
the other antemolars, i.e. to the persistent dentition. 
(2) Teeth corresponding to the permanent set in other mammals are 
present in rudiment, but are not developed, with the exception of the 
third premolar. 
(3) Leche’s “ enamel germs ” are what he believed them to be, and 
seem to be rather progressive than retrogressive. 
(4) A prelacteal dentition is represented not only in the front part 
of the jaw, but also beside the molars. In short, the author corroborates 
Leche’s position. 
Development of Teeth in Rodents.* — Herr Paul Adloff has in- 
vestigated the jaws of the embryos of sixteen different rodents by means 
of serial sections, in order to determine the homology and succession of 
the teeth. The result is to show that the large incisors correspond to 
the second incisors of the typical dentition, a conclusion which was pre- 
viously arrived at by Cope on palaeontological grounds. In the order 
as a whole two processes have been going on hand-in-hand : — (1) the 
specialisation of the large incisors, and (2) the reduction of certain other 
teeth. In the most primitive forms, the Sciuromorphae, the embryonic 
dentition is as follows : — 
ld x — Id 3 Cd P^ Pd 9 Pd 3 ) 
M, M 2 M 3 
— i 2 — — - p 3 ! 
- I, - - - — | 
• Mj M 2 M 3 
Id, Id 2 — Cd — Pd 2 Pd 3 I 
As noticed by Leche in other mammals, the lower jaw shows more 
specialisation than the upper as regards reduction, so that dental germs 
are present in the upper jaw which are quite absent from the lower. 
The one exception to this is, that in the lower jaw there are prelacteal 
rudiments which do not occur in the upper, and also a rudimentary pre- 
decessor of the large incisor which is not represented in the upper jaw. 
Of the prelacteal rudiments, two are especially distinct, one near Pd 2 , 
and another near Pd 3 . The latter fuses with Pd 3 and helps to form this 
well-developed tooth. This result leads the author to question Messrs. 
Wilson and Hill’s interpretation of the homologies of the dental germs 
* Jenaische Zeitschr. Nalurwiss., xxxii. (1898) pp. 317-110 (4 pla. and 4 figs.), 
T 2 
