UUI'LINES OF THE DEVELOPMENT OF THE TUATARA. 45 



present on each side in the mesoblast of the amnion, just above 

 the line where the latter joins the body of the embryo (fig. 80, 

 J5. v.). These vessels probably join the vitelline veins in 

 front, but of this I was unable to make certain, owing to im- 

 perfection of some of the sections. In two embryos of this 

 stage the heart was observed to be beating. 



One histological feature at this stage seems to deserve 

 notice. In the hinder part of the body the epiblast (fig. 81, 

 Ep.) is very conspicuously thickened on the flanks of the 

 embryo, being divided into distinct inner and outer layers, 

 widely separated but connected by strands of protoplasm con- 

 taining a few nuclei. 



Stage L (figs. 83—91). 



To this stage I refer four embryos, viz. : 



47 Collected ou Stephens Island Nov. 30, removed from egg Dec. 16. 

 50 „ „ „ 30, „ „ 27. 



82 „ „ eud of Nov., „ „ 27. 



o* ,, ,j ), ), )> "(• 



The separation of the serous envelope has extended widely, 

 so that it can be readily torn off nearly all round the yolk-sac 

 as a thin transparent membrane, but it is still adherent to the 

 true amnion above the embryo about the region of the shoul- 

 ders, at the spot marked with an X in fig. 83. 



The living embryo is still almost transparent except for the 

 presence of the red blood, and the beating of the heart was 

 observed in three cases. 



The external characters of the embryo are shown in figs. 83 

 and 84. Not only is the down-bending of the fore-brain very 

 strongly marked at this stage, but there is also a strong ventral 

 flexure in the region of the neck and shoulders, so that the 

 head end of the embryo hangs down into the yolk, enveloped 

 in the pro-amnion. A well-marked flexure in the opposite 

 direction has also made its appearance in the lumbar region. 



At the hinder extremity of the body the short tail (figs. 83, 

 85, T.) projects freely into the pleuro-peritoneal cavity between 

 the yolk-sac and the serous envelope, and in No. 50 (fig. 85) 



