HEREDITY. 403 



perfectly ossified. This evidence, it appears to me, 

 effectually disposes of the question of the production 

 of these structures during the lifetime of the individ- 

 ual. They are as truly inherited as is the number of 

 digits or any other important organ in the animal econ- 

 omy." 



Such observations may be repeated indefinitely. 

 Thus the astragali of the higher Mammalia are already 

 grooved before birth, and are not flat up to that tirtie 

 as in their Puerco ancestry. The reduction of digits 

 appears very early in foetal life, and the ball and socket 

 articulations of the cervical vertebrae of the Diplarthra 

 are by no means introduced after birth. 



The teeth possess the normal structure of their 

 crowns while yet in the alveoli before eruption. In 

 some cases the transition from a primitive to a modern 

 type of tooth has been observed to take place in the 

 embryo. 



Dr. A. von Brunn^ has shown that in the embryos 

 of the rat, the enamel-producing epithelial layer of the 

 molar teeth undergoes a remarkable change at the 

 places where the transverse crests of the crowns are 

 to appear. Before the enamel layer is deposited, the 

 portion of the epithelial layer corresponding to the 

 cross-crests undergoes degeneration, as a result of 

 which it acquires the character of a stratified squamous 

 epithelium. Thus no enamel is laid down on the sum- 

 mits of the cross-crests, which present the exposed 

 dentine when erupted. Now it is a fact that the crowns 

 of the molar teeth of the ancestors of the genus Mus, 

 were covered with enamel at maturity, like all other 



1 " Notiz tiber unvollkommene Schmelzentwickelung auf den Mahlzahnen 

 der Ratte, Mus decumanus ' ' : ArchivfUr Mikrosko^ische Anatomic, 1880, XVII, 

 pp. 241-343, PI. XXVII. Ryder, American Naturalist, l888,' p. 547. 



