424 GASTRULATION 



The body of the streak serves as the "door" through which migrating meso- 

 dermal cells other than the cells of the pre-chordal plate-notochordal area 

 pass from the epiblast layer downward to the space between the epiblast and 

 hypoblast. 



Using the Vogt method of vital staining, Pasteels ('37b) was able to demon- 

 strate morphogenetic movements of cells into the primitive streak area and 

 thence to the inside similar to that described by Spratt (fig. 202G-I). 



The evidence derived from the carbon-particle-marking technic and that of 

 vital staining, therefore, strongly suggests that the primitive streak of the chick 

 forms as a result of: 



(a) converging movements of the epiblast cells toward the median Une of 

 the posterior half of the pellucid area and 



(b) cell proliferation in situ within the streak. 



3) Cell Movements in the Hypoblast and the Importance of Those Move- 

 ments in Primitive-streak Formation. The hypoblast or entodermal layer of 

 the blastula appears to play a significant role relative to the formation of the 

 primitive streak in the bird. Various lines of evidence point to this conclusion. 

 For example, Waddington ('33) reported the results of experiments in which 

 he separated the epiblast from the hypoblast of early chick and duck embryos 

 in the early, primitive-streak stage. He then replaced the two layers so that 

 their longitudinal axes were diametrically reversed, that is, the anterior part 

 of the entoderm (hypoblast) lay under the posterior part of the epiblast, while 

 the posterior part of the entoderm lay below the anterior region of the epiblast. 

 The following results were obtained: 



(1) The development of the original streak was suppressed; or 



(2) a new, secondary, primitive streak was induced. 



During later development, in some cases, the secondary streak disappeared; 

 in others, it persisted and a double monster was produced. In other instances 

 the primary primitive streak disappeared and the secondary streak persisted. 

 The general conclusion set forth by Waddington is as follows: the entoderm 

 does not induce the differentiation of a definite tissue, but rather, it induces 

 the form-building movements which lead to the development of the primitive 

 streak. 



Certain experiments made by Spratt ('46) lend added evidence of the im- 

 portance of the hypoblast in primitive-streak formation. In eight experiments 

 in which the hypoblast was removed before streak formation, six cases failed 

 to produce a streak, whereas in two instances a beginning streak was formed. 

 It may be that in the latter two cases, the induction of morphogenetic move- 

 ments within the epiblast cells occurred previous to hypoblast removal. These 

 experiments are too few to permit a definite conclusion; however, they are 

 suggestive and serve to bolster the conclusion made by Waddington. In a 



