OUTLINES OF THE DPIVELOPMENT OF THE TUATARA. 21 



Oil either side the cells of tlie primitive streak divide into 

 two distinct layers, an upper layer which joins the columnar 

 epiblast of the area pellucida, and a lower layer which forms 

 part of the lateral sheet of mesoblast running out into the sur- 

 rounding blastoderm. 



Between this lateral sheet and the columnar epiblast, espe- 

 cially adherent to the under surface of the latter, a few irre- 

 gular mesoblast cells are clearly recognisable. These cells are, 

 I think, undoubtedly derived from the original lower layer. 

 It will be seen by reference to figs. 12 — 14 that the epiblast, 

 with these few adherent mesoblast cells, is separated by a dis- 

 tinct interval from the deeper or primitive-streak mesoblast. 

 This separation appears to be the commencement of the split- 

 ting off of the serous envelope (S. En.) which will be described 

 in later stages (compare figs. 30, 31, 55). 



The hypoblast {Hyp.) forms an ill- defined and possibly im- 

 perfect layer of flattened cells, closely adherent to the under 

 surface of the primitive streak, and probably also derived, as 

 in the front part of the embryo, from the original lower layer 

 of the blastoderm. 



The section represented in fig. 13 passes actually through 

 the opening of the blastopore {B. P.), which is seen as a wide 

 funnel-shaped depression on the upper surface of the primitive 

 screak, lined by distinctly columnar cells. 



Fig. 14 represents a section through the primitive streak 

 behind the blastopore, and embraces a somewhat wider area 

 than the preceding figures, so as to show a portion of the area 

 opaca and its junction with the area pellucida. The yolk-laden, 

 sub-embryonal membrane, which forms the floor of the seg- 

 mentation cavity beneath the area pellucida, is not represented. 

 It will be seen that the blastoderm of the area opaca (A. 0.) is 

 very much thicker than that of the area pellucida {A. P.) owing 

 to the enormous development of the lower layer with its 

 contained yolk-spheres. In this region of this particular 

 embryo the thickened lower layer of the area opaca passes 

 quite suddenly into the thin lower layer of the area pellucida, 

 forming a kind of " germinal wall ^' {G. TV.), but the transition 



