THE DEVELOPMENT VF LKPIDOSI KKK PARADOXA. 



425 



arrangement is to be found in the fact that in the young 

 individual the teeth are furnished with definite prominent 

 pointed cusps — each probably representing the tip of an 

 originally simple denticle^— although in ontogeny they de- 

 velop as a perfectly continuous ridge from the beginning. 



t/i.. 



Text-fig. 1.— Sa<?ittal section llirough bead region of a Protopicrus 



larva (Stage 33). Cam. Zeiss a*, oc. 4. 



b.c. Buccal cavity, d.p. Dental papilla. ej>. Pineal body. 



p. Paraphysis. th. Thyroid rudiment. 



The first obvious rudiments of teeth occur about Stage 30 

 in the form of a thickening of the oral epithelium,i under 

 which the mesoblast becomef* concentrated as it were, the 

 nuclei being crowded much more closely together than 

 elsewhere. 



By Stage 31 the thickening is growing downwards into 

 the mesoblast so as to border on each side a ridgo-like 

 " papilla " of mesoblast (PI. 26, fig. 7 a). 



The first traces of hard structure in the tooth appear about 

 Stage 32, when a conical calcareous cap appears beneath the 



I For general topographical rclalions of this sec Tcxt-(ig. 1. 



