THE FORMATION OF THE ORGANS THE ALIMENTARY CANAL. 201 



yielded by. the lower epithelium (Fig. 91 A). The large groups of 

 tooth-forming cells (odontoblasts) form a kind of cushion or bed upon 

 which the teeth are modelled (Fig. 91 A and B). In the shape of 

 this cushion, the future form of the tooth is already shown. In the 

 Opisthobranchia and the Pulmonata, a special differentiation occurs, 

 only a few (four to five as seen in longitudinal section) * very large cells 

 undertaking the formation of one tooth (Fig. 91 B, od)'; the most 

 anterior of these large cells is said to yield the part of the basal 

 membrane that underlies the tooth now in course of formation. 



Fi<;. !*1. .1 uud B, Longitudinal sections through the radular sac of Octopus 

 <-it/</<t,-ix (A) and Helix memoralis (B) (alter H^SSLER). bm, basal membrane; od, 

 odontolilasts ; , ep, upper epithelium; sn, sub-radular membrane; it, ep lower 

 epitliflium ; z, teeth. 



The tooth thus produced fuses with the basal membrane and with 

 tlu- prolongation of the basal part of the last tooth (Fig. 91 B). 

 When H tooth is thus completed, this cell-group undertakes the 



* [There are in reality eight to ten of these large cells concerned in the secre- 

 tion of each tooth, the cells being arranged in two parallel series, so that, in a 

 longitudinal section, like that shown in Fig. 91, only one row is seen at a time. 

 It is probable that the three most posterior pairs of these odontoblasts secrete 

 the main body and hook of the tooth, the next transverse pair secreting the 

 base, while the most anterior pair secretes the sub-radular membrane. ED.] 



