CASTRULATION IN VARIOUS CHORDATA 



425 



second set of experiments performed by Spratt, chick blastoderms in the pre- 

 streak and early-streak stages were inverted and marked with carbon particles. 

 The results showed that the hypoblast moves forward in the median line below 

 the epiblast layer. He also demonstrated that this forward movement of the 

 hypoblast "precedes the anterior differentiation of the primitive streak." Spratt 

 further observed that: When the movement of the hypoblast deviated to the 

 left or to the right, the primitive streak similarly deviated. This evidence 

 "strongly suggests that the hypoblast influences the development of the primi- 

 tive streak in the overlying epiblast" (Spratt, '46). 



AREA PAC A 



PE L LUCIDA 



EMBRYONIC 



SHIELD 



MIDPRIMITIVE STREA 



^^;t^. - PROAMNION ARE 



,,'''hEAD PROCESS 

 ,-i-J* INOTOCHORD ) ..•^■-.y, . ,i^,'5iVv 



'1^ FMHRYONIC AREA ~\LI:MS^k.i' 

 -PRIMITIVE 



.^M— ■ i -Ih/W ^ EXTRA-EMBRYON 





.^^., ,. ^1 . , 9-p R I M I T I V E G R 



- ''; '' ~-,'^. ' ^^ ■ — PRIMITIVE FO 



}'^l-^^.Miii^iai!^^^:i£i 



MATURE PRIM 

 STREAK 



E AD-PROC ESS 

 STAGE 



Fig. 203. Surface-view drawings of photographs of developing primitive streak. (From 

 Hamburger and Hamilton, '51, after Spratt.) (A) Initial streak, short, conical thick- 

 ening at posterior end of blastoderm. (Hamburger and Hamilton, '51, stage 2.) (B) 

 Intermediate streak. Thickened streak area approaches center of area pellucida. (Ham- 

 burger and Hamilton, '51, stage 3.) (C) Definitive streak (average length, 1.88 mm.). 

 Primitive groove, primitive fold, primitive pit, and Hensen's node are present. (Ham- 

 burger and Hamilton, '51, stage 4.) (D) Head-process stage (19 to 22 hours of incu- 

 bation). Notochord or head process visible as area of condensed mesoderm extending 

 anteriorly from Hensen's node. Proamnion area is indicated in front portion of area 

 pellucida; head fold is not yet present. (Hamburger and Hamilton, '51, stage 5.) 



