GASTRULATION IN VARIOUS CHORDATA 421 



caudal region of the area pellucida. A median, sagittal section through a 

 comparable stage is shown diagrammatically in figure 171 A. Figure 203 A 

 illustrates an early beginning streak normally found eight hours after incuba- 

 tion of the egg is initiated, while figure 203 B presents a medium streak, ap- 

 pearing after about 12 to 13 hours of incubation. In figure 203C, a definite 

 primitive streak appears in which the primitive groove, primitive pit, primitive 

 folds, and Hensen's node (primitive knot) are outlined. This condition occurs 

 after about 18 to 19 hours of incubation. This may be regarded as the mature 

 streak. A later streak after about 19 to 22 hours of incubation is indicated 

 in figure 203D. Observe that the head process or rudimentary notochord 

 extends anteriorly from Hensen's node, while the mesoderm is a deeper-shaded 

 area emanating from the antero-lateral aspect of the streak. The clear proam- 

 nion region may be observed at the anterior end of the area pellucida. In the 

 proamnion area, mesoderm is not present at this time between the ectodermal 

 and entodermal layers. 



2) Cell Movements in the Epiblast Involved in Primitive-streak Formation 

 as Indicated by Carbon-particle Marking and Vital-staining Experiments. 

 Recent experiments by Spratt ('46), using carbon particles as a marking de- 

 vice, have demonstrated that epiblast cells from the posterior half of the pre- 

 streak blastoderm gradually move posteriad and mediad as gastrulation pro- 

 ceeds (figs. 202, 204, black arrows). Before the actual appearance of the 

 streak, mesodermal cells begin to appear between the epiblast and hypoblast 

 at the posterior margin of the area pellucida. (See fig. 202B, involuted meso- 

 derm). As cellular convergence posteriorly toward the median line continues, 

 the primitive streak begins to form as a median thickening posteriorly in the 

 peUucid area (fig. 202C, observe posterior median area indicated in white). 

 The rudimentary primitive streak formed in this manner gradually advances 

 anteriorly toward the central region of the pellucid area of the blastoderm 

 (fig. 202D, E). In the thickened area of the developing primitive streak, shown 

 in white at the posterior median portion of the blastoderm in figure 202C, 

 there are about three to four cell layers of epiblast together with about the 

 same number of layers of mesoderm below. At its anterior end the streak is 

 thinner. 



The anterior end of this early streak gradually grows forward as a result 

 of cell proliferation in situ and by cells added through convergence of cells 

 from antero-lateral areas (Spratt, '46). Some of the cells at the anterior end 

 of the forming streak may involute or ingress from the epiblast into the space 

 between the hypoblast and epiblast and thus come to lie at the anterior end 

 of the forming streak, while other cells ingress laterally between these two 

 layers (fig. 202C-E, K-O). 



As the streak differentiates anteriorly by addition of cells to its anterior 

 end, it also elongates posteriorly by cellular additions to its caudal end. The 

 carbon-marking experiments of Spratt demonstrated further that, during the 



