170 H. SPENCER HARRISON. 



made, and embryos from the fifth to the ninth or tenth montli 

 of incubation may all be inchided in his Stage R. 



The teeth of the first series have become k'ss tooth-like in 

 form, and some of them are enclosed in the epidermis (fig. 

 4, pal. 1). They are not so numerous as in the last embryo 

 described, and perhaps some of them have already been shed. 

 They occur as far back in the jaw as the ninth maxillary 

 teeth of the future functional series. 



As regards the future functional teeth, there are in each 

 p rem ax ill a three teeth, the first being smaller and less 

 developed than the other two, and the second being the 

 most advanced. 



Each maxilla has ten teeth, and, as in the last embryo, 

 these exhibit an alternation similar to that already described, 

 but in this case I am able to give a more detailed description. 

 Before doing so I wish to refer to fig. 11, where the 

 alternating series of the maxilla at Stage S is shown. Here 

 a large tooth alternates with a small one, and the larger differ 

 somewhat in shape from the smaller. A comparison of tigs. 

 6 and 7 will illustrate the condition found in frontal 

 sections at Stage R. Fig. 6 shows a tooth having a consider- 

 able development of dentine, a lingual prolongation of the 

 dental lamina (?•. d. I.), stellate tissue much broken down and 

 only separated from the exterior by two or three layers of 

 flattened cells. Fig. 7, on the other hand, shows a tooth in 

 which the amount of dentine is much less, though the enamel 

 organ as a whole is larger; there is no lingual prolongation 

 of the dental lamina ; the stellate tissue is more intact, and is 

 separated from the exterior by several layers of cells; the 

 whole structure is obviously in an earlier stage of deve- 

 lopment. Fig. 6 is the eighth maxillary tooth of the right 

 side, and fig. 7 is the tooth immediately anterior to it ; the 

 former represents one of the smaller teeth of the alternating- 

 series shoAvn in fig. 11, the latter one of the larger. This 

 alternation of character, as seen in sections, prevails through- 

 out the maxillary series, and it is difficult to avoid 

 coming to the conclusion that we are here dealing 



