592 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
or forms in which both sets are mingled. The fact is also against the 
theory that the milk dentition is a secondary acquisition. Perhaps the 
abortion of the secondary set may be connected with the diet of the 
toothed Cetaceans. The baleen whales should be ranked in a distinct 
order, without close genetic relation with the toothed forms. That they 
too were originally heterodont seems to Kiikenthal to be indicated by 
the occurrence in the embryos of peculiar composite teeth like molars. 
The author pronounces strongly against the theory which would 
connect Cetaceans with seals through Zeuglodon. He also discusses the 
dentition of Marsupials, which his embryological investigations (on one 
species) have led him to regard as a first, not as a second set. Only one 
tooth of the rudimentary second set comes to development. Everything 
seems to Kiikenthal to point to the conclusion that the oldest Mammals 
were diphyodont. Both sets are equally primitive. 
Kukenthal’s general theory of the evolution of teeth may be sum- 
marized as follows. The lowest grade is that of Elasmobranchs where 
the dermal denticles are diffuse, but attain special development on the 
jaws, and are replaced without limit. The second grade occurs in Am- 
phibians and Reptiles, in which the teeth are not diffuse, but restricted 
to the jaws, limited in number and in replacement. The third grade is 
that of Mammals, in which of the lateral rows of teeth, only the two 
well-known sets persist. With the shortening of the jaws the conical 
teeth become fused in the original multituberculate molars, of which the 
lines of specialization are now known with some definiteness. 
Development of Rodent Teeth.* — Herr P. Freund corroborates the 
observations of Chabry and Pouchet as to the occurrence on the upper 
and lower jaw of Lepus of a rudimentary tooth in front of the large 
incisor. He inclines to regard it as a much reduced milk-tooth corre- 
sponding to the large incisor. Herr Freund finds in the diastema on 
the upper jaw of Lepus a well-developed dental ridge without enamel- 
germs. This ridge is indistinct in Cavia , absent in Cricetus and Mus. 
In Sciurus , however, the ridge is uninterrupted, and, besides Stenson’s 
duct, there are two closely apposed enamel-germs, of which the more 
anterior is the more distinct. These are perhaps vestiges of posterior 
incisors or of canines. 
Pcecilogony.t — M. A. Giard returns to this subject, J being drawn to 
it by the recent observations of Messrs. Brook and Herrick on the meta- 
morphosis of the Macroura.§ He points out that poecilogony shows us 
in the clearest way by what processes all extended embryogeny becomes 
abbreviated. From the taxonomic point of view the phenomena are 
also very important ; a number of species of Insects are based on dif- 
ferences in development. In species of pcecilogonic origin the differen- 
tiation is not such as to prevent a certain amount of successful crossing ; 
in converging species, when the distinction has existed for a long time, 
crossing will be sterile or impossible. For example, the different species 
of Typhlocyba, of the section T. rosse , are in a state of forced amixia, 
owing to the great differences in the copula tory organs ; and yet these 
* Arch. f. Mikr. Anat., xxxix. (1892) pp. 525-55 (2 pis.). 
f Comptes Rendus, cxiv. (1892) pp. 1549-52. 
j See this Journal, 1891, p. 332. § See ante , p. 476. 
