638 The American Naturalist. [August, 
fications of the ectoderm described by Klaatsch, we have noth- 
ing to do here further than is concerned in the scale develop- 
ment. It is to be noticed that along with the formation of the 
little patches of scleroblasts the overlying cells of the basal 
layer become elongated, the first step in the development of an 
enamel organ. The later stages in their general features are 
much as described by Oscar Hertwig in his classic paper of 
twenty years ago, and yet there are important differences to be 
noted. The heaping up of the scleroblasts continues, the re- 
sult being the formation of the dentine organs, carrying with 
it the superposed enamel cells in the form of a pyramid. The 
enamel organ is terminated on all sides by a groove, and even 
at this stage the cells at the bottom of this groove are actively 
engaged in proliferating additional scleroblasts which are 
pushed into the still-growing dentine organ. The necessary 
conclusion is not only is the enamel of the placoid scale an . 
ectodermal derivation, but such is the nature of the dentine as 
well. 
Now placoid scales and teeth have long been regarded as 
homologous structures, and so Klaatsch studies the history of 
the latter. In the sharks he finds that the conditions of the 
development of the scales are paralleled in the ontogeny of 
the teeth. There is the same early proliferation of scleroblasts 
into the clear layer, and later, when the enamel cap is formed, 
its limiting groove is the seat of additional ingrowth of den- 
tine-forming cells. In short, we must no longer regard the 
teeth as structures derived from two germ layers—ectoderm 
and mesoderm—but as purely ectodermal products. 
In the fin of the shark are numerous horny rays, and their 
history is followed. Earlier workers had universally regarded 
them as belonging to the connective-tissue series, although in 
1885 Krukenberg had shown that their organic base was differ- 
ent from the chemical standpoint from the other connective 
tissues. Klaatsch finds that here there is a similar inwander- 
! It is to be noted that in the recent meeting of the Anatomische Gesellschaft at 
Strassburg, May 13-16, Professor Rabl had a paper “ Ueber die Herkunft des Den- 
tinkeims in den Placoidschuppen und den Zihnen der Selachier (gegen 
