BONES, JOINTS, CONNECTIVE TISSUES 37 



The greatest difference found between the measurements of 

 the skulls for the two sexes is in the nasal bones, which are 

 nearly 2 per cent longer in the male skull. The greater relative 

 length of the nasal bones in the male may be regarded as a sec- 

 ondary sexual character (Hatai). 



a) Teeth. Addison and Appleton ('15) report as follows on 

 the size and growth of the incisor teeth in the Albino. 



The dental formula of the albino rat is 



4 o-: M M i 



There is only one set of teeth, and hence the dentition is mono- 

 phyodont. The time of eruption of the various teeth extends 

 over a period of 3| weeks. The incisors are the first to appear, 

 viz., at 8 to 10 days after birth. The first and second molars 

 erupt at about the 19th and 21st days respectively, and it is 

 after this latter period that the young animals may be weaned 

 and are able to maintain an independent existence, as far as food 

 is concerned. The third molars are delayed until two weeks 

 later and do not appear until about the 35th day. 



The incisors are permanently-growing (or rootless) teeth, while 

 the molars have a definite limited period of development and 

 acquire roots. A wide diastema separates the incisors from the 

 molars as may be seen by reference to figure 1 (loc. cit.) The 

 incisors are strongly curved and Owen (1840-1845) has described 

 the lower incisor as being the smaller segment of a larger circle, 

 and the upper incisor as the larger segment of a smaller circle. 

 In the case of lower incisor of the albino rat this statement 

 needs a slight modification. 



The times of the early stages of development of the incisors 

 were as follows: 



14 day fetus — slight thickening of oral epithelium. 



15 day fetus — distinct thickening and growth inwards of oral epithelium. 



16 day fetus — dental ledge and beginning of flask-shaped enamel organ. 



17 day fetus — dental papilla with crescentic enamel organ capping it. 

 19 day fetus — both ameloblasts and odontoblasts differentiated. 

 Xew-born animal — enamel and dentine formation begun. 



8 to 10 days — eruption of the tooth. 



