48 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



or elliptical or pear-shaped with its narrow end posterior, becomes 

 distinguishable from the rest of the blastoderm by its slightly greater 

 opacity. The area in question is known as the embryonic shield 

 (e.s), and its opacity is due to its ectoderm being thickened, the 

 individual cells having taken on a columnar form. 



Either enclosed within or projecting beyond the posterior outline 

 of the embryonic shield (Fig. 29, B) is a small area in which there is. 



Fig. 29. — Illustrating gastrulation in the Gecko (Platydactylus). (After Will, 1892.) 



A, showing complete blastoderm with the embryonic shield in the centre. This is bounded behind 

 by the gastrular rim, precociously developed in this specimen. B, embryonic shield of specimen at the 

 stage in which the archenteric floor is breaking down. C, embryonic shield at later stage where gas- 

 trular rim is bent back into a A-shape bounding the yolk-plug: the outline of the mesoderm sheet is 

 seen on each side. D, embryonic shield showing stage at which the gastrular lips have come together 

 so as to bound a longitudinal slit, b.d, edge of blastoderm; e.s, embryonic shield; g.l, gastrular 

 lip ; vies, limit of mesoderm. 



no layer of columnar ectoderm sharply marked off from the lower 

 cells. This forms the primitive plate (Fig. 31, p.p). The boundary 

 of the embryonic shield gradually spreads outwards and the primitive 

 plate comes to be, if it is not already, enclosed within it. 



Within the limits of the primitive plate the surface of the egg 

 now becomes involuted to form a groove bounded anteriorly by a lip 

 which from its correspondence with what we have seen in lower 

 forms, more especially in the Gymnophiona, is clearly to be recog- 



