OUTLINES OF THE DEVELOPMENT OF THE TUATARA. 51 



The anterior half of the embryo begins to withdraw from 

 the invaginated yolk-sac, but the extent to which this takes 

 place is not at all constant. Thus in No. 103 (fig. 96) the 

 withdrawal seems hardly to have commenced, while in No. 93 

 (figs. 97, 100) it has progressed so far that the fore-limb has 

 become completely extricated. 



In No. 92 the yolk attached to the under surface of the 

 yolk-sac appears to be spreading inwards, so as to reduce the 

 clear area surrounding the embryo (fig. 97) ; but in No. 103 

 (fig. 96) the clear area is as large as in the preceding stage. 

 The attached yolk is more or less strongly corrugated. 



The embryo, as its anterior end withdraws from the yolk-sac, 

 comes to lie on its left side, and the allantois, now large and 

 with a well-developed circulation, passes up on the right side 

 and comes to lie above the embryo, between it and the serous 

 envelope (fig. 100). 



The alimentary canal has now become completely closed in 

 ventrally, except for a small aperture (figs. 97, 98, 0. Al.) 

 where the splanchnic stalk passes out to the yolk-sac. 



The limbs have elongated, and both manus and pes have 

 become evident (fig. 98), but no digits are yet visible, at any 

 rale externally. 



The tail has lengthened considerably, and is coiled inwards 

 in a close spiral on the ventral surface between the hind limbs 

 (fig. 98, T.). 



The visceral clefts have begun to close up, but the hyoman- 

 dibular (figs. 96, 98, H. M.) is still conspicuous externally. 



On the under surface of the head a broad fronto-nasal 

 process (fig. 99, F. N. P.) is present, but it does not yet 

 meet the large superior maxillary process {S. 31.) on either 

 side, so that the external nares (Na.) are not yet closed in 

 behind. 



The mid-brain is very prominent (figs. 98, 99, ilf. jB.), and 

 just in front of it the parietal eye and its *' stalk " are clearly 

 visible in the unstained embryo examined as an opaque object, 

 though as yet without pigment. The " stalk " appears in the 

 middle line at the hinder end of the roof of the thalamence- 



