248 BULLETIN OF THE 



The jaws of a sheep embryo whose length was 37 m.m. were sec- 

 tioned so that the plane of the first section was parallel to the median 

 plane of the jaw. After a small number of sections had been cut par- 

 allel to this plane, the object was slightly rotated, so as to keep the 

 plane of the sections nearly perpendicular to the outer margin of the 

 jaw. By thus frequently changing the plane of cutting, each section 

 of the series was a true cross-section of the part of the jaw from which 

 it was taken. This method of cutting must give more satisfactory 

 results than can be had by cutting either parallel or perpendicular to 

 the median plane of the jaw without any change in direction. 



In the first of the sections of the upper jaw, in the region which 

 corresponds to the first incisor of the lower jaw (fig. 1), the dental la- 

 mina is not distinct from that portion of the buccal epithelium which 

 Pouchet and Chabry ('84, p. 152) have called the "mur plongeant." 

 But before the first incisor of the lower jaw has been passed, and in the 

 region of the second and third incisors (figs. 2, 3, and 5), the dental 

 lamina, although not sharply marked oft" from the " plunging wall," is 

 readily distinguishable from it. By comparing figs. 1 and 3, one finds 

 that the epithelial tissue in fig. 3 sinks deeper into the mesoderm than 

 in fig. 1 ; that the deep portion forms an angle of about 45° with the 

 axis of the plunging wall in fig. 1 ; and that the width of this deep 

 portion, that is, the distance from the base of the malpighian layer on 

 one side, to the base of the same layer on the other side, is less than 

 the width of the plunging wall in fig. 1. In fig. 3 the dotted line is 

 introduced to show where the boundary of the plunging wall, as it 

 appears in fig. 1, would fall if it were " projected " on to the plane of 

 this section. This deep portion beyond the dotted line, although con- 

 tinuous with the plunging wall, and not histologically different fi'om it, 

 I believe to be, both by reason of its size and its direction, the most 

 anterior indication of the dental lamina. In the sections through the 

 canine region of the same jaw (fig. 7) the conditions of the dental la- 

 mina and plunging wall are the same as in the incisor region. 



Dursy ('69) gives drawings of several sections through the incisor 

 and canine regions of a sheep embryo at about the same stage as the 

 one just described. He says (1. c, p. 214) : " Taf. III. Fig. 1-8 zeigt 

 die Zahnfurche mit dem Schmelzkeim von einem Schafsfotus, dessen 

 Gaumen im Schliessungsprocess begriffen war ; auffallend daran ist die 

 Weite dor Furche und daher auch die Breite der hellen Kernmasse des 

 •Schmelzkeims an alien Schnitten, die somit an den zahnlosen Stellen 

 beim Schafe um diese spate zeit noch vorhanden ist." Dursy evidently 



