24 ARTHIIW DRNDY. 



The rudiment of the pericardium, observed in the preceding 

 stage, appears to be almost obliterated by stretching in the 

 formation of the pro-amnion. It seems to be represented by 

 the split, containing a few mesoblastic cells, which lies between 

 the epiblast and hyboblast, where they turn up to join the 

 pro-amnion just in front of the opening into the anterior closed- 

 in part of the alimentary canal (fig. 24). 



Having thus described the condition of the foetal mem- 

 branes, we may consider next the external features of embryos 

 of this stage. 



The appearance of embryo 56, when viewed from above as 

 an opaque object without staining, is represented in fig. 15. 

 The embryo lies close to the hinder end of the pear-shaped 

 area pellucida, the outline of the anterior portion being some- 

 what dimly seen through the overlying blastoderm. At the 

 hinder end, just in front of the primitive streak, lies the large 

 crescentic opening into the amniotic cavity (Am. 0.) On 

 each side the primitive streak runs out into a wing of meso- 

 blast {L.JV.), which at once turns sharply forward and runs 

 for a while almost parallel with the long axis of the embryo, 

 but dies out before reaching the middle of its length. Of 

 course the mesoblast also extends backwards from the primitive 

 streak, and the latter, with its posterior and lateral mesoblastic 

 outgrowths, forms a crescentic thickening around the hinder 

 end of the embryo which corresponds more or less closely to 

 the " sickle " of other reptilian embryos.^ 



Fig. 16 represents the same embryo viewed from beneath 

 after removal of the sub-embryonal membrane, a fragment of 

 which, however, remains just behind the embryo {S. E .M.). 

 The anterior half of the embryo is now, of course, very much 

 more distinct, and is seen as a finger-shaped body, curving 

 somewhat downwards from its junction with the posterior half, 

 and projecting freely forwards beneath the blastoderm enve- 

 loped in its thin transparent pro-amnion. The mesoblastic 

 outgrowth from the primitive streak appears now to form a 

 conspicuous thickening all round the posterior half of the 

 ' Compare Mitsukuri and Isliikawa (5). 



