8 THÉEL, DEVELOPMENT OF ECIIINUS MILIARIS L. 



and the wotidce:> make their appearance. From the structure 

 of these calcareous pieces which constitute tlie beginning of 

 the fnture dental apparatus, it becomes evident that tkey 

 arise from two different origins.^ The halfpyramids, epiphyses 

 and rotnlpe have their own centres of calcification and com- 

 mence their development, like other larval and postlarval 

 parts of the skeleton, as very minute calcareous grains which 

 rapidly grow to small stars with three very short arms. 

 These small three-armed bodies gradually change into perfo- 

 rated plates etc. Subseqnently they grow larger and thicker, 

 and finally combine in order to form the »Aristotles lantern». 

 I never fonnd that the coelom or membranes derived from it 

 partook in the formation of the calcareons parts mentioned. 

 In my opinion they tirst originate from cells which have 

 wandered in between the tissues. For the present, however 

 I am obliged to leave it undecided whether the peripharyngeal 

 membrane partakes in the låter develojjment of the pyramids 

 or not, throngh the former supposition does not seem 

 probable. 



As to the tooth itself, it has evidently quite a different 

 origin. My own experiences have led me to the opinion 

 that it is developed from the ectoderm, a theory which may 

 possibly prove to be wrong. The fact is that it is very 

 difficult to decide whether an organ arises from the one or 

 from the other of two close-lying embryonal tissues. 



For the present, my conviction is that each tooth takes 

 its origin at the bottom of five interradial narrow tubular 

 pouches of the ectodermic disc and that these pouches, forming 

 folds at their blind upper end, constitute the calcifying matrix 

 of the tooth. Thus, according to my opinion, the tooth should 

 be equivalent to the enamel in the vertebrated animales. 



As may be understood from the following quotations 

 these views do not agree with those expressed by MacBride. 

 He says: » At 27 — 28 days the rudiments of the masticatory 

 system of the socalled »Aristotle's lantern» make their appea- 

 rance. These are five outgrowths of the left posterior coelom, 

 having the shape of inverted wedges, which alternate with 

 the five radial canals which have been developed from the 



^ Comp. Lovén"s Echinologica (Bihang K. Svenska Vet.-Akad. Hand- 

 lingar. 18. IV. 1892) and Théel's Echinocvamus pusillus (Royal Soc. of Sciences 

 — Upsala 1892). 



