ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
33 
Development of the Occipital Segment.* — Prof. Cb. Debierre has 
studied this in man and mammals, and emphasises the essential 
similarity of tlie occipital region in all cases, its intracartilaginous 
origin, its composition out of four parts which remain separate or fuse 
in variable degree, and various other facts. He traces the relation of 
the embryonic metameres to the adult skull; but his work is corroboratory, 
not novel. 
Abnormalities in Human Development-! — Prof. C. Giacomini adds 
other cases to the series of teratological phenomena which he has de- 
scribed, and proposes the following classification : — 
I. Those in which there is an embryo which retains its relations 
with the membranes. 
a. Atrophic forms, in which the internal and external struc- 
ture is profoundly modified, but without obliteration 
of organs. 
(3. Nodular forms, in which microscopic examination shows 
no trace of organs. 
II. Those in which the embryo is wanting. 
a. In which the embryo has disappeared in situ , leaving 
(a) all the foetal annexes, or (h) all but the chorion. 
(3. In which the embryo has left its cavity, passing (a) from 
the amnion into the external coelom, (6) away from all 
the membranes, (c) away from all but the chorion, 
( d ) producing destruction of the chorion. 
Dentition of Mammals.! — Prof. W. Leche has completed the first 
part of his extensive memoir on the development of the teeth in 
Mammals. He has investigated 27 different genera , often in 7-11 stages. 
We shall restrict our reference to some of his general results. 
The dental groove stands in no causal relation to the origin or de- 
velopment of the teeth, but is a transitory epithelial structure. So the 
lip-groove has nothing directly to do with the enamel ridge, and any 
union of the two is secondary. The rudiments of teeth and of skeletal 
parts are quite independent ; thus teeth rooted in the premaxiila may be 
homologous -with teeth in the maxilla. The enamel or dental ridge — a 
unified ridge of epithelium — is the beginning of tooth development ; a 
distinction of primary and secondary ridges cannot be upheld ; a stage 
with free papillae is very rare. The ridge causes an elevation on the 
free surface of the buccal epithelium, but immediately grows down into 
the mesoderm, and where it goes deep enough causes a thickening 
therein. A proliferation, exclusively or mainly to the labial side, forms 
the enamel-germ which passes through bud-like, cap-like, and bell- 
shaped stages. At the last stage, as the enamel pulp arises, the germ 
begins to be constricted off from the ridge, and this emancipation is a 
necessary condition for the origin of a new enamel germ, and the more 
important the free end of the enamel ridge, the greater is the predis- 
position to form a new tooth. Only when the end of the ridge is swollen 
into a club or bud and surrounded by a dental sac, can a realised rudi- 
* Joum. de l’Anat. Physiol., xxxi. (1895) pp. 385-426 (2 pis.), 
t Atti R. Accad. Sci. Torino, xxx. (1895) pp. 642-63. 
X Biblioth. Zool. (Leuckart and Chun), Heft 17, 1895, 160 pp. (19 pis.). 
1896 d 
