OUTLINES OF THE DEVELOPMENT OF THE TUATARA. 73 



two being sharply distinguished from one another though re- 

 maining in close contact. The wall of the optic cup divides 

 into two primary layers, in the inner of which pigment is de- 

 posited, while the outer becomes connected with the brain by 

 the special nerve of the parietal eye, which is not formed from 

 the so-called stalk. The so-called " stalk " of the parietal eye 

 probably represents the right parietal eye retarded in develop- 

 ment by the overlapping of the left half of the roof of the 

 fore-brain, and in an extremely degenerate condition. 



The Ears. — The development of the auditory organs, so far 

 as followed, is quite normal. The auditory pits appear about 

 Stage J as shallow depressions of the superficial epiblast on 

 either side of the hind brain, the epiblast being here com- 

 posed of a single layer of elotigated columnar cells (figs. 58, 

 64). These pits gradually deepen (fig. 79), and about Stage L 

 they entirely close up, forming two sacs, each still lined by a 

 single layer of columnar cells (fig. 91, Au.). Further than 

 this their development has not been followed. 



The Olfactory Organs. — The nasal pits first appear as 

 shallow depressions of the superficial epiblast at about Stage 

 L (fig. 83, Na.). They have the usual crescentic outline, open 

 towards the mouth (figs. 92, 94, Na.). A.s development goes 

 on they gradually deepen, and with the downgrowth of the 

 fronto-nasal process, commencing about Stage O (fig. 99), the 

 external nares become defined in the usual manner as two cres- 

 centic apertures the margins of which are completed at Stage Q 

 (fig. 103). At Stage R a very remarkable feature makes its 

 appearance, the nostrils being completely plugged up by a 

 dense cellular mass derived from proliferation of their epiblastic 

 lining. The external nares now appear from the outside as 

 two small lound white spots (Fig. 104, Na.), being filled up to 

 the level of the surrounding epidermis with the plug of cells 

 just mentioned. This remarkable plugging up of the nostrils 

 takes place about the commencement of the long winter rest 

 which the embryo passes through at Stage R, and probably 

 bears some relation to this process of hibernation. I have no 

 material sufficiently well preserved to enable me to state when 



