THK DKVKLOI'MKNT OF LKI'I DoSl H RN I'AIIADOXA. 427 



tory to breaking' through. The enamel layer is closely fused 

 with the underlying dentine; the sharp line separating the 

 two has completely disappeared, and it is only possible, by 

 the use of very high powers, to distinguish the enamel by its 

 clear appearance witliout any obvious structure from the 

 dentine, which still shows a faint reticular or fibrillar struc- 

 ture — the remnants of the more obvious structure of the 

 same kind in the un calcified odontoblast. 



Finally, in Stage 38 (fig. 7 p), when the young Lepidosiren 

 has already begun to feed, the teeth have broken freely 

 through the oral epithelium, the enamel organ having disap- 

 peared entirely except for a vestigial flap (fig. 7 f, e. o.) stick- 

 ing up all round the base of the tooth. The enamel is now 

 no longer to be detected in my sections : it has probably been 

 worn off, being doubtless, from its larger proportion of 

 organic matter, much less hard than ordinary enamel. The 

 mass of dentine has much increased in size. A little later its 

 central portion assumes the hard glassy character of the 

 vitrodentine of the adult (" Enamel," Tomes' ' Dental 

 Anatomy,' fifth edition, p. 263). 



Hypophysis. 



The hypophysis is somewhat obscure in Lepidosiren. 



In Stage 23+ it is visible as a somewhat wedge-shaped in- 

 gro\vth of the deep layer of the epiblast. 



In Stage 29+ the deep end of the structure has become 

 slightly swollen, with indications of a longitudinal split in its 

 middle; the portion connecting this with the ectoderm is 

 thinned down to a narrow thread occupying the space between 

 the closely approximated front end of the gut and the floor of 

 the fore -brain. 



At a stage very slightly later (30) the connecting istlimus 

 is nipped through, while the expanded extremity, whose 

 split is now widening out into a definite cavity, lies as a 

 closed sac beneath the infundibulum. 



