48 EMBRYOLOGY OF THE LOWER VERTEBRATES 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 



B 



e.s. 



, e.s. 



Tries 



I/,,;. 29. Illustrating gastrulation in the Gecko (Platydactylus). (After Will, 1892.) 



A, showing complete blastoderm with the embryonic shield in the centre. This is hounded 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 su-e where gas- 

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



M-i-n on each side. |), embryonic shield showing stage at which the -astrul.-ii lips hav ( i,- to-rthei 



BO as to bound a longitudinal slit, h.ti, edge of blastoderm; ..,. embryonic shield; ././, gastrnlai 

 lip ; HIKS, limit of mesoderm. 



no layer of columnar ectoderm sharply marked nil' f'n>in the lower 

 This forms the primitive plate (Fi#. :'>!, 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 e-,u 

 now becomes inv..lut.i-(| to form a groove bounded anteriorly l>y a lip 

 which from its correspondence with \\hat \\e have seen in lower 

 forms, more especially in the ( lymnophiona, is clearly to he n 



