76 



NORTH AMERICAN FAUNA 74 



Fetiis, 178 mm. 18 August (L stained and cleared; R serially sec- 

 tioned). — Primary follicle formula 



. 1-2-3 1 1-2-3-4 1 



Calcification centers were present in all follicles except those of i^, i2, pi, and the 

 molars. In the serial sections, primordia of all of the secondar>' teeth, except I^, 

 I2, and Pl, were identified; that of the upper secondary canine already showed a 

 well-differentiated enamel organ. The diastema between 13 and cj was not 

 apparent, but that between c^ and p2 was wide. Primordia of secondary molars 

 were present; as in man and other mammals, however, these apparently do not 

 develop beyond the state of undifferentiated laminae. 



Fetus, 186 mm, 20 August (L stained and cleared; R serially 

 sec-tioned). — Primary follicle formula 



. 1-2-3 1 1-2-3-4 



c-rP-i7^m-rx 2 = 32. 



1-2-3 1 ^ 2-3-4 1 



Calcification centers were present in all but i^, i^, pi, and m^. The alveolus of i^ 

 in the left tooth row only was bisected by a slender ridge of bone, forming an 

 elongate, double alveolus, as is often seen in the loci of the rudimentary sec- 

 ondary incisors of older specimens. The lower molars were not detected in the 

 serially sectioned right half of the head, but the alveolus of m^ was present in the 

 stained and cleared left half. 



Fetus, 208 mm, 10 September (stained and cleared). —FrimaT}' follicle 

 formula 



. 2-3 1 1-2-3-4 1 ^ _ 



Calcification centers w^ere present in all primar}' follicles except those of i^, pi, 

 p-^, and the molars (Fig. 49, left). The alveolus of ci was more elliptical in cross 

 section than in the younger specimens and extended farther posteriad. It was 

 continuous with that of pi and separated from the alveolus of p- by only a 

 slender ridge of bone. The follicle of io was situated in a hemi-alveolus adjacent 

 to the mandibular s>'mphysis; that of 13 was smaller and shallower than in am' of 

 the previous specimens. The diastema between c^ and po was still open on the 

 right but closed on the left. 



Fetus, 285 mm, 4 October (stained and cleared). — Primary follicle formula 



• 3 1 1-2-3-4 1 ^ _ 

 c— p ^ ^ . m— X 2 = 30. 



1-2-3 1 ' 2-3-4 1 



Except for pi, p4, and the molars, each follicle contained the calcified crown of a 

 tooth (Fig. 49, right). The follicle of pi was much smaller than those of the other 

 premolars and ' vsls not appreciably larger than in the 106- and 132-mm 

 specimens. The follicle of cl had attained the shape and alignment seen in older 

 specimens. In the normal positions of il and i2 were several elongate slots in the 

 alveolar bone, similar to those in the locus of i^ in the 186-mm fetus. In the right 

 lower tooth row, the follicle of i3 was larger than that of Cj; on the left it was 

 smaller than c^. The latter seems to be the usual condition. The diastema 

 between c^ and p2 was closed, bilaterally. 



