ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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fine branches, which pass into the fibrils of the surrounding neuroglia. 
As to the function both of the dendrites and of the fine outgrowths of 
the nerve-cells, he believes that these constitute the nutritive apparatus 
of the cells. 
Teeth of Reptiles.* — Dr. Hugo Levy has investigated the structure 
and development of the teeth in reptiles, especially in several species of 
Lacerta. He finds that the teeth consist largely of dentine, but are 
furnished with a thin cap of enamel. The enamel is essentially similar 
to that of mammalian teeth, but the prisms are represented only by fine 
lines in the homogeneous ground-substance. The dentine, as in other 
animals, consists of a matrix traversed by numerous canaliculi, whose 
arrangement differs in different reptiles. As to the development of the 
teeth, the first indication of their appearance is found in the insinking of 
the innermost layer of the mucous membrane to form the dental ridge. 
While this is in progress, the connective tissue proliferates rapidly in 
certain areas which mark the germs of the future teeth, and in these 
areas becomes almost surrounded by ectoderm. In this process the 
author believes that the connective tissue papillae play an active part, 
and are not merely passively surrounded by the ectoderm cells. Con- 
trary to the conditions which prevail in mammals, these tooth-germs 
become largely differentiated before separation from the dental ridgo. 
The successional teeth arise either from offshoots of the primitive dental 
ridge, or from certain nests of epithelial cells produced by the breaking 
up of the primitive dental ridge. These nests of epithelial cells seern 
to owe their origin to the connective tissue which interpenetrates the 
dental ridge, and thus divides it up into separate portions. The enamel 
of these, as of other teeth, the author believes to be a cuticular product 
of the epidermal cells, impregnated with lime, after a fashion which 
is quite analogous to that seen in the cuticle of the Crustacea. The 
enamel membrane he also regards as merely a cuticular product. 
Similarly he considers the dentine to be a secretory product of the 
odontoblasts. 
Teeth of Gadidse.f — Mr. C. S. Tomes has made a study of the differ- 
ences in the minute structure of the teeth in this family. That there 
should be great diversity in the dentitions is intelligible enough when 
we consider how much the members of the family differ in respect of 
food, habits, size, and external form; “but it is not so easy to under- 
stand how the group of influences knowm as natural selection should 
have operated in the direction of producing differences of minute struc- 
ture ; for it would seem as though it mattered little what the histologic il 
structure of a tooth might be, as far as the exercise of its functions goes, 
so long as it is sufficiently strong, sufficiently sharp, and of an appro- 
priate shape.” The author’s general conclusion is against any mechani- 
cal theory (kinetogenesis) of the evolution of tooth forms. “ For if the 
form of a tooth is the direct consequence of the direction of stimulation 
that it has received by use in successive generations, then a tooth which 
is subject to the very minimum of use, such as that of the gill-rakers of 
the hake, ought not to be so exact a copy of the teeth round the margins 
* Jenaische Zeitschr. Naturwiss., xxxii. (1898) pp. 313-46X1 ph). 
t Quart. Journ. Micr. Sci., xli. (1899) pp. 459-69 (1 ph). 
