GASTRULATION IN VARIOUS CHORDATA 



423 



formation of the streak up to about the condition present at 20 to 22 hours 

 of incubation (figs. 2021, K; 203D), almost the entire posterior half of the 

 pellucid area, consisting of presumptive pre-chordal plate, notochord, and 

 mesoderm, is brought into the streak and involuted to the inside between the 

 hypoblast and epiblast (figs. 202F-H; 204). This condition of development 

 is often referred to as the "head-process stage" (stage 5, Hamburger and Ham- 

 ilton, '51 ). At this stage the approximate, antero-posterior limits of the future 

 embryonic body of the chick, exclusive of the extra-embryonic tissue, are 

 shown by the general area beginning just anterior to the head process and ex- 

 tending for a short distance posterior to Hensen's node (figs. 203D; 205D, E). 

 As indicated in figure 202, there are two parts to the primitive streak: 



( 1 ) the area of Hensen's node and primitive pit concerned with invagi- 

 native movements of pre-chordal plate mesoderm and notochordal 

 cells and 



(2) the body of the streak. 



The former area appears to arise independently in the center of the pellucid 

 area, while the body of the streak is formed at the median, caudal margin of 

 the pellucid area, from whence it grows anteriad to unite with Hensen's node. 



NOTOCHORD \ 



PRIMITIVE PIT \ 



Fig. 202. Migration of cells during gastrulation in the chick. Drawing to the left of 

 the midline represents a surface view; to the right of the midline the epiblast layer has 

 been removed. (A-F) To the left of the midline based on data provided by Spratt, '46. 

 (J) Represents lateral, sectional view of (F)-(G), viewed from the left side. Arrows 

 indicate direction of cell migration. (K)* Indicates a left lateral view of (I), with the 

 epiblast cut away midsagitally throughout most of the left side of the blastoderm. (L-O) 

 Transverse sections of (K). as indicated on (K). 



