FATE OF BLASTOPORE, ETC., IX CHELOXIA. 



Woodcul II. 



-sp 



-sp 



Sections through the ijriuiitive streak of a 

 Gecko embryo of the Vlllth Stage, according to 

 Will {Zool. Centralbl. I. p. 137). 



A 12 sections behind the anterior lip of the 

 blastopore. w-%c, central field ; 7/-u', peri- 

 pheral field. 



2 sections behind the anterior lip of the 

 blastopore. The i^eripheral field is fused 

 with the ectoderm of the embryonic 

 shield and is no longer distinct. The 

 central field appears as the plug between 

 the lips of tlie primitive grove, pr. 



B 



Woodcut III. 



'/ ^ rf B 



Diagrams showing the changes taking place 

 in the primitive plate, according to Will (Zool. 

 Centralbl. I. jh 138). w,]Aa,stoi->ore; rf, peripheral 

 field ; mf, central field. 



the primitive plate i.-; sharply 

 marked oi¥ from the siir- 

 roiiDcling- epiblast as shown 

 at y, y, Woodcut II. A. The 

 diagrammatic surface-view of 

 the ])i'imitive plate would 

 therefore be an area circum- 

 scrihed by a distinct line all 

 around, as in AVoodcut III. 



A, The blastopore (Invagina- 

 tionsoifnung), which is a 

 narrow transverse slit in this 

 figure, now begins to assume 

 a crescentic shape, and its 

 horns then gradually extend 

 backwards over the entire 

 primitive plate (Woodcut III. 



B. C. D.), nntil they meet in 

 the posterior median line. Thus 

 an elliptical central field 

 (Mittelfeld) is circumscribed 

 by a ring-like peripheral field 

 (Randfeld), the line of de- 

 markation between the two 

 fields being formed by the 

 blastopore and its posterior 

 horn -like prolongati(3ii on 

 each side (Woodcut III.). As 

 the peripheral field is marked 

 off from the central field, the 



