149 
The dental lamina at various stages. Whilst the alter- 
nating teeth are being formed on the dental laminae, the latter be- 
come involved in epidermal ingrowths of a different nature, the result 
being that in the more advanced embryos it is not possible to define 
their limits. Soon after hatching, however, we can again in certain 
regions distinguish them from the superposed ingrowths, which 
have now assumed the form of glandular grooves, lying lingually 
to the teeth (Fig. 6). The new dental laminae only appear at the 
anterior portions of the dentigerous bones, in the regions, that 
is to say, where successional 
teeth are to be formed (Fig. 6). MII 
This latency of the dental | a 
laminae in epithelial ingrowths wo} 
of a different nature appears Gato [iwc 
to be quite peculiar to Hat- 
teria. In the later months of 
incubation also, the alternating 
teeth have unique relationships 
with the labio-dental strands. 
Fig. 6. Transverse section through 
lower jaw, showing developing tooth of 
4th dentition (M. IV), situated lingually 
to one of the smaller teeth of the alter- 
nating series (M. IT). It will be seen 
from the figure that the amount of bone 
about to be thrown out with the deci- 
duous teeth is very great — it is ex- | 
ceptionally great in this case. M.IV 
at a later stage displaces 5 or 6 of the £523 
alternating series, and becomes the main lem. ee ] 
constituent (the “canine”) of the large . ) 
front “tooth”, d. dentary. Mek Meckeli 
cartilage. (Young specimen 21,2 cm long. / A 
X 30.) N 
Mck Der 
The Successional Teeth (Dentitions IV and V). 
These commence their formation some months after hatching, 
when the animal is about 15 cm in length. They arise on the new 
dental laminae and are restricted to the anterior regions of the tooth- 
bearing bones (see Figs. 2,3,4 and 6). It isevident from the figures 
that the later dentitions are being suppressed. The process is tra- 
velling from behind forwards, tooth replacement being most com- 
pletely retained in the premaxillae, and entirely lost in the posterior 
members of the alternating series, The tendency appears to manifest 
