426 



GASTRULATION 



Fig. 204. Movements in the epiblast layer of the chick during gastrulation and 

 primitive-streak formation. (Modified sHghtly from Spratt, '46.) (A) Pre-streak con- 

 dition. Carbon particles are placed as indicated, at a, b, c. d, e, f, and g. (B G) Observe 

 migration of carbon particles. (C) Short streak. (E) Medium broad streak. (G) Long 

 streak. (See fig. 203C.) 



4) Primitive Pit and Notochordal Canal. If one compares the notochordal 

 canal, formed during gastrulation in the reptilian blastoderm, with that of 

 the primitive pit in the chick, the conclusion is inevitable that the primitive 

 pit of the chick blastoderm represents an abortive notochordal canal. The 

 lizard, turtle, and chick thus represent three degrees of notochordal canal 

 development (figs. 200A, D; 202J). In certain birds, such as the duck, a 

 notochordal canal very similar to that of the turtle gastrula, is formed. 



5) Resume of Morphogenetic Movements of Cells During Gastrulation in 

 the Chick. In view of the foregoing facts relative to primitive-streak forma- 

 tion, steps in the gastrulative procedure in birds may be described as follows: 



(a) Shortly after the incubation period is initiated, hypoblast material at 

 the caudal end of the blastula starts to move in the median line toward 

 the future cephalic end of the embryo. This activity may be regarded 

 as a gastrulative streaming of the hypoblast. (This streaming move- 

 ment probably represents the chick's counterpart of the forward move- 

 ment of the entodermal area in the dorsal-lip region of the frog 

 embryo. ) 



(b) After this movement of the hypoblast is inaugurated, cells from the 

 epiblast layer immediately overlying the moving hypoblast pass down- 

 ward toward the hypoblast. That is, epiblast cells begin to involute 

 and come to lie between the epiblast and hypoblast; from this new 



