Development of the Tuatara (Sphenodon punctatum). 441 



account of the development with numerous illustrations. As, how- 

 ever, it will still take some time to complete the drawings and manu- 

 script, it appears desirable to publish at once a short summary of the 

 most interesting results obtained. The general development, as 

 already stated by Thomas, conforms closely to that of other reptiles, 

 but the following features seem to deserve special mention : 



(1) The development occupies about thirteen months, the eggs 

 being laid (on Stephen's Island) in November and hatched about 

 midsummer of the year following. The last stages in the develop- 

 ment, after about the first four months, occupy a much longer period 

 than the earlier ones, so that, having reached an already very 

 advanced stage, the development seems to be almost if not quite 

 suspended during the winter months. 



(2) The blastoderm spreads around the yolk at a very early date, 

 and the embryo first appears as a cap-shaped mass of cells, the front 

 end of which is elevated above the surrounding blastoderm as the 

 head-fold, while the hinder and narrower end is formed by an undif- 

 ferentiated mass of cells representing the primitive streak. The 

 front part of the embryo is formed of epiblast and lower layer cells, 

 and from the lowest of the latter the hypoblast is subsequently 

 differentiated. 



(3) In the primitive streak a distinct blastopore makes its appear- 

 ance, which presently opens into the enteron below, forming a very 

 distinct neurenteric canal which persists for some time. 



(4) The notochord appears to be formed by a forward growth from 

 the primitive streak in front of the blastopore, rather than by dif- 

 ferentiation of hypoblast cells in the mid-dorsal line of the enteron. 



(5) At a very early date the front end of the embryo sinks into the 

 yolk, pushing the subjacent blastoderm before it in such a manner 

 that the latter forms a kind of amnion closely investing the head and 

 the thoracic portion of the body. This "amnion," though very 

 thin, becomes differentiated into inner somatopleuric and outer 

 splanchnopleuric portions, but, at any rate for a long time, without 

 any mesoblast. 



(6) At a comparatively late stage in development the anterior 

 end of the embryo, together with the somatopleuric layer of the 

 " amnion," is withdrawn from the splanchnopleuric layer (which 

 belongs really to the yolk sac), and thus the embryo comes to lie 

 entirely above the yolk sac. 



(7) In the hinder part of the embryo the amnion is formed by 

 uprising folds of somatopleure meeting and fusing above the embryo, 

 probably accompanied by a down sinking of the embryo. This 

 process is continued backwards for some distance behind the embryo, 

 forming a narrow canal which communicates in part with the cavity 

 of the true amnion, and opens behind on the surface of the blasto- 



