OUTLINES OF THE DEVELOPMENT OF THE TUATARA. 35 



side of tlie body of the embryo, fonuiiig a kind of blister (com- 

 pare figs. 89 — 71). Along the mid-thjrsal line, however, the 

 body of the embryo is attached by the true amnion to the 

 under surface of the serous envelope (compare figs. 68 — 70), 

 except just above the primitive streak, where the two have 

 completely separated (fig. 71). 



The FcBtal Membranes. — As we have just seen, the 

 serous envelope (fig. 69, >S. En.) has completely split off" from 

 the underlying portion of the blastoderm (yolk-sac) throughout 

 the posterior half of the 8-shaped clear area, while in front of 

 this it is not yet widely separated. It is composed as before 

 of the highly characteristic but now somewhat flattened cells 

 of the superficial epiblast, with their very darkly staining 

 nuclei, and of a number of adherent mesoblast cells derived 

 from the lower layer, some of which appear to be specialising 

 as a distinct ej)ithelioid layer on the under surface of the thin 

 transparent membrane thus formed (figs. 72, 73, Mes. E.). 

 In the mid-dorsal line of the embryo, from about the level of 

 the vitelline veins to the primitive streak, the serous envelope 

 remains continuous with the true amnion (figs. 68 — 70). 

 Above the primitive streak, however, it lies quite separate 

 above the true amnion, so that the two lateral halves of the 

 pleuro-peritoneal space communicate freely above the embryo 

 in this region (fig. 71). They also extend backwards behind 

 the embryo as a single wide cavity, roofed over by the serous 

 envelope and bounded by the sinus terminalis. On the floor 

 of this pleuro-peritoneal cavity, formed by the yolk-sac, the 

 vitelline vessels are rapidly developing in the form of blood 

 islands (figs. 69, 72, B. /.), which togetiier with the sinus ter- 

 miualis [S. T.) are, as already noticed, probably derived from 

 the sheet of mesoblast which spreads out from the primitive 

 streak. 



The pro-amnion, completely enveloping the anterior end of 

 the embryo in front of the vitelline veins, has exactly the same 

 structure and relations as before, being a thin transparent mem- 

 brane composed of two layers, — an outer hypoblastic, which 

 is really a portion of the yolk-sac, and an inner epiblaslic, 



