A. M. Spurgin 77 



stratum intermedium. lie described the l)uds for the permanent teeth 

 as arising from the outer layer of the enamel organ, and not, as Tomes 

 has shown it in one of his figures, as coming from the mucous mem- 

 brane of the mouth cavity. The bud for the eighth tooth, which has 

 no predecessor, he described and illustrated as arising directly from 

 the mucous membrane of the mouth cavity. With the exception of the 

 first, the teeth are bicuspid. The tooth-buds of two rudimentary incisors 

 were described but no dentine had been deposited. Eose found the same 

 general condition in the embryo of Dasijpiis liyhridus. Besides two rudi- 

 mentary incisors, there were seven back teeth, the first two having 

 single cusps. On the whole the development wa« further advanced than 

 in the other embryo, dentine having been deposited in the rudimentary 

 incisors as well as in some of the back teeth. Rose found in connection 

 with the enamel organ of both of these rudimentary incisors, secondary 

 buds coming from the outer epithelial layer, the one from the second 

 incisor being best developed. He remarks that this does not cut off 

 the possibility that tliis tooth may also have a successor in the later 

 change of teeth. He states that while the embryos examined by him 

 bad no enamel, they did have, as a secretion product of the enamel cells. 

 a thin structureless membrane lying directly against the dentine and 

 exactly corresponding to the formation which in other animals we call 

 Nasymth's membrane. 



Ballowitz examined two embryos of Dasypns novemcinctiis of 6 and 

 8 cm. respectively. He found a typical enamel organ, with inner and 

 outer epithelial layers, stratum intermedium, and well developed stel- 

 late reticulum. He describes the processes of the inner columnar 

 epithelial cells, generally known as Tomes' processes, but says he has not 

 been able to explain them. He states that very soon after the first layers 

 of dentine have been deposited, the outer layer of cells disappears and the 

 stellate reticulum is replaced by connective tissue. He says that while 

 it is true that the inner epithelial layer and stratum intermedium remain 

 over the calcified dentine in an unbroken layer, they have undergone a 

 considerable change ; the inner layer loses its columnar shape and becomes 

 flattened, the stratum intermedium is reduced, so that only two or three 

 layers of flat cells can be found on the cusps. Whether these cells have 

 anything to do with the development of Xasymth's membrane, or Avhether 

 in these teeth such a membrane was present at all, Ballowitz says, it 

 was impossible to decide. In the tooth-buds of the larger embryo, which 

 were separated only by connective tissue, he found secondary buds coming 

 off from the lingual side of the outer epithelial layer of the enamel 

 organ. Xo rudimentary incisors were described, and I presume none 



