OUTLINES OF THE DEVELOPMENT OF THE TUATARA. 49 



There are (in No. 51) forty-one mesoblastic somites. 



The limbs appear as outgrowths of the Wolffian ridges, the 

 fore-limb {F.L.) opposite to mesoblastic somites 9 — 13 inclusive, 

 the hind limb {H. L.) opposite to somites 28 — 32. 



Still only three visceral clefts are visible externally in the 

 opaque embryo, but the visceral arches between them are 

 much more prominent than in the last stage, and the superior 

 maxillary process {S. M.) has begun to grow out. 



The nasal pits {Na.) are much larger and more distinct. 



The liver {Liv.) begins to form a conspicuous projection just 

 behind the heart. 



The proctodseum has formed as an invagination of the 

 epiblast on the ventral surface immediately behind the 

 allantois. 



The primary parietal vesicle still remains as a simple sac 

 opening into the cavity of the fore-brain, slightly to the left of 

 the middle line. Its position is again marked, P. V,, fig. 92, 

 by a projection between the paired eyes. 



Stage N (figs. 94, 95). 



To this stage I refer four embryos, numbered 93 — 97, all 

 collected on Stephens Island about the end of December, 1897, 

 and removed from the eggs on January 6th, 1898. 



No. 96 (fig. 94) may be taken as typical of this stage, 

 which is distinguished from the preceding by the following 

 features : 



The curvature of the body has increased somewhat. 



The limbs are much more conspicuous, and the distal 

 extremity of the fore-limb {F. L.) has become flattened to form 

 the hand. 



The tail is larger, and more distinctly twisted into a spiral. 



The allantois is larger, and has become vascular. 



All four visceral clefts are visible externally when the embryo 

 is examined as an opaque object. 



The superior maxillary process {S. M.) is much larger. 



The pigment in the paired eyes is for the first time con- 



VOL. 42, PART 1. — NEW SER. D 



