大黒鼠の下顎第三臼齒の形状に就て : クマネズミ屬下顎臼齒の進化學説に對する新考察
スポンサーリンク
概要
- 論文の詳細を見る
The shape of the molars of albino rat (Rattus norvegicus albus, Fitzinger 1867) has been said to resemble extremely to those of black rat (Rattus rattus, Linnaeus 1758) or brown rat (Rattus norvegicus, Erxleben 1777). But the authors, by close observation, could recognize a new fact in the shape of the lower third molar of albino rat. Tooth crown of the lower third molar of albino rat consists of two laminae, one of them is the anterior and the other is the posterior. The anterior lamina is divisible into two main tubercles, namely, the internal and the external tubercles. A small teret tubercle, namely, supplemental tubercle is present at the anterio-external border of this lamina. So far the appearances are similar to those of black or brown rat. The posterior lamina, in our observation however, also shows faint indication of division into two main tubercles with a small supplemental tubercle as in the case of the anterior lamina (See Pl. I). This detection about the above-mentioned condition of the posterior lamina of the lower third molar of albino rat is our new knowledge. To give an outline of the customary explanation about the lower third molar of a rat, instead of the albino rat, as there is no description concerning to the evolution of molars, especially of albino rat, G. S. Miller (1912) said that the lower third molar corresponds to the part of the third lamina and the posterior heel of the lower first molar which is the typical form among lower molars. According to his opinion, the posterior lamina of the lower third molar is similar to the posterior heel of the first molar, but considerably larger. (See Pl. II, Fig. 14). Against this idea, I. Koyama (1926) stated that the lower third molar corresponds to the part of the first and the second laminae of the first molar, and to the posterior lamina of the lower third molar corresponds to the second lamina of the first molar (See Pl. II, Fig. 13). On the other hand, K. Watanabe (1937) stated that the lower third molar corresponds to the part of the vestige of the second lamina, the third lamina and the posterior heel of the first molar. (See Pl. II, Fig. 12). Our close observation shows that the shape of the lower third molar of the albino rat is different far from these foregoing descriptions with respect to the shape of the crown and relation to the roots. Now, comparing with the shape of the lower first molar which is the typical form among lower molars, the anterior lamina of the lower third molar corresponds to the second lamina of the first molar, while the posterior lamina of the lower third molar, we inclined to believe, as the union of the third lamina with the posterior heel. Therefore, we consider that the lower third molar is composed of the same elements as those of the lower first molar without the first lamina. In other words, the elements of the third molar are quite equal to those of the lower second molar, viz., the lower third and the lower second molars are homological organs. (See Pl. II, Fig. 11). Furthermore, we are of the opinion that the above fact is not only true in the case of the albino rat but it is also true in case of the black or brown rat.
- 社団法人日本動物学会の論文
- 1940-07-15