Dendy. — Life-history of the Tuatara. 251 



eggs swell considerably, doubtless by absorption of moisture 

 through the leathery egg-shell. I attribute this absorption of 

 moisture to the hygroscopic action of a semi-gelatinous fluid 

 which is secreted in large quantity in the cavity of the 

 enormously developed allantois. Finally, a very high state of 

 tension is reached, when a very small incision through the 

 egg-shell is sufficient to cause it to split open with almost 

 explosive violence, as I twice experimentally determined. 



In the later stages of development a patch of horny 

 epidermis on the snout of the young animal forms a sharp 

 cutting instrument, which is doubtless used for the purpose of 

 making the necessary incision in the egg-shell. A similar 

 structure may be observed in the chick and in numerous other 

 vertebrate embryos. 



The embryos obtained form a very perfect series, and have 

 been classified and described under sixteen stages, distin- 

 guished by the letters of the alphabet from C to S. For a 

 detailed description of these stages, with the necessary illus- 

 trations, I must refer the reader to my monograph on the 

 development. It will suffice here to give a very brief sketch 

 of the life-history. 



The earlier stages closely resemble those of the tortoise, 

 especially as regards the development of the fcetal membrane 

 known as the amnion. This structure, as in other verte- 

 brates, surrounds the embryo in the form of a bag or sac as 

 it lies on the surface of the yolk. In the Tuatara this sac is 

 continued behind the embryo for some distance, in the form 

 of a long narrow canal or tunnel, open posteriorly. Such a 

 structure as this "posterior amniotic canal " was entirely 

 unknown in any animal until a few years ago, when it was 

 discovered by the Japanese embryologist, Professor Mitsu- 

 kuri, in the embryos of a tortoise. Since the publication of 

 the summary of my observations in June last Professor 

 Mitsukuri has kindly sent me a copy of his work, and it is 

 really extraordinary to see how closely his drawings of the 

 tortoise embryo agree with mine of the Tuatara. Another 

 feature in which the young embryos of the Tuatara agree 

 closely with those of the tortoise is the curious manner in 

 which the head dips down into the yolk, enveloped in a 

 peculiar membrane known as the " pro-amnion." 



In general features, however, the earlier stages of the 

 development closely resemble those of other vertebrates with 

 heavily yolked eggs. The formation of the alimentary canal, 

 the four pairs of gill-slits in the neck, the central nervous 

 system, the eyes, ears, and nostrils, the notochord, the limbs, 

 &c., takes place in the manner already well known in other 

 types. 



At stage E, however, during which the hibernation occurs, 



