76 TEXT-BOOK OF EMBRYOLOGY. 



embryo as does the blastopore, and that in some birds the primitive pit 

 opens into the archenteron in the same manner as the blastopore. It has 

 already been pointed out that the caudal margin of the sector where the blas- 

 toderm has been reduced to the thickness of one layer of cells was rolled or 

 tucked under when gastrulation began, and that the germ wall was lacking 

 along this margin. It was also stated that as gastrulation proceeded the 

 two ends of the germ wall approached each other and eventually met behind 

 the margin of the sector, and that the two horns of the crescentic groove 

 were withdrawn toward the median line and finally closed (Fig. 43). Imme- 

 diately after these phenomena the primitive streak appears, extending for- 

 ward from the center where the horns of the crescent were drawn in and closed. 

 It would seem therefore that the formation of the primitive streak is a con- 

 tinuation of the gastrulation process. 



Head process Neural plate 



Ectoderm 

 Mesoderm 

 ^l Entoderm 



Yolk cell 



FIG. 48. Transverse section of blastoderm of chick (21 hours' incubation). Hertwig. Section 

 through head process, anterior to Hensen's node. 



In Fig. 46 there can be seen the slightly opaque band extending for- 

 ward from the primitive streak which has been designated the primitive axis 

 or head process. In cross section (Fig. 48) it is obvious that the opacity is 

 due to the fused mass of entoderm and mesoderm, while the ectoderm here is a 

 separate layer. In a longitudinal section which includes both axis and streak 

 (Fig. 49) the ectoderm is observed to fuse with the other two layers at the 

 anterior end of the streak. It is probable that the primitive axis is not the 

 result of a forward growth from the end of the streak, but is the result of 

 the separation of the ectoderm from the other two layers from before back- 

 ward. That is, if one imagines the primitive streak at its full development 

 before the axis has appeared, and then imagines a wedge started just beneath 

 the ectoderm and driven backward, one can readily see that the ectoderm 

 will be separated from the underlying and still fused mesoderm and entoderm. 

 As this continues the axis thus becomes longer. The streak does not become 

 correspondingly shorter, however, because it increases at the caudal end; 

 in other words, as the primitive axis increases in length the primitive streak 

 recedes or is carried backward by additions to its own organization. This 



