OUTLINES OF THE DEVELOPMENT OP THE TUATAHA. 11 



which period it appears that a considerable number perish 

 naturally from drought. 



(3) The earlier stages in development are passed through 

 much more rapidly than the later, and, Stage R having been 

 reached in March, the embryo makes little further progress 

 during the winter months, and does not hatch until about the 

 middle of the following summer (January). 



There is only one other animal known to me, the eggs of 

 which take such a long time to develop, from the time of 

 laying to the time of hatching, and that is the oviparous 

 Victorian species of Peri pat us, described by me under the 

 name of Peripatus oviparus (2).^ As I have noticed else- 

 where, an egg of this animal which was laid in my vivarium at 

 Melbourne took no less than seventeen months to develop 

 before hatching. Sphenodon and Peripatus are both 

 doubtless extremely ancient types, which have persisted with 

 but little modification for a very long period, and one is strongly 

 tempted to connect this fact with the extraordinarily long period 

 occupied in their development from the egg. 



(c) On the Structure of the Egg. 



The eggs of the Tuatara are oval in shape, usually about 

 equally rounded at the two ends, but sometimes rather narrower 

 at one end than at the other. They vary very considerably in 

 dimensions,^ and the size of the egg seems to bear no relation 

 to the stage of development of the contained embryo. Thus, 

 taking the measurements of eight eggs numbered consecutively 

 as they came in, and all containing embryos belonging to the 

 same stage (R), I found them to be as follows : 



• Compare the observations of Mitsukuri and Isliikawa (5) on the similar 

 variation in size of the eggs of Trionyx. 



- Cf., however, a note iu 'Nature' (vol. Iviii, p. 619) by Mr. G. A. 

 Boulenger, F.R.S., on the embryo of Emp. orbicularis, contributed while 

 this paper was iu the printers' hands. Although most probably due to a 

 different cause, the " suspended gestation " of the roe deer may be cited as a 

 kindred phenomenon (Bischoff, ' Entwick. des Eehes. Giessen,' 1884, and 

 the ' Zoologist,' 1889, p. 86). 



