GASTRULATION IN VARIOUS CHORDATA 433 



b. Gastriilation in the Pig Embryo 



In the pig embryo, two centers of activity are concerned with the forma- 

 tion of the primitive streak, namely, a caudal area of mesodermal proliferation 

 which forms the body of the primitive streak and an anterior primitive knot 

 or Hensen's node. The similarity of behavior of these two portions of the 

 primitive streak in the chick and pig suggests strongly that their formation 

 by a convergence of superficial epiblast cells occurs in the pig as it does in 

 the chick. Hensen's node, originally described by Hensen (1876) in the rabbit 

 and guinea pig, is a thickened area of the epiblast in the midline near the 

 middle of the embryonic disc. As in the chick, the body of the primitive streak 

 takes its origin at the caudal end of the embryonic disc, where the first appear- 

 ance of the streak is indicated by a thickening of the epiblast (fig. 209A, B). 

 From this thickened region, cells are budded off between the epiblast and 

 hypoblast, where they migrate distad as indicated by the lightly stippled areas 

 in figure 208. The streak ultimately elongates, continuing to give origin to cells 

 between the hypoblast and epiblast. Eventually, the anterior neck region of 

 the body of the streak merges with Hensen's node (fig. 208E, F). From the 

 anterior aspects of the primitive (Hensen's) node, cells are proliferated off 

 between the epiblast and hypoblast, and a depression or pit, the primitive pit, 

 appears just caudal to the node. 



The proliferation of cells from the nodal area deposits a median band of 

 cells which merges anteriorly with the hypoblast below. More caudally, the 

 hypoblast becomes attached to either side of the median band of cells (fig. 

 209C). The median band of nodal cells thus forms part of two regions, viz., 

 an anterior, pre-chordal plate region, where the nodal cells are merged with 

 hypoblast (entoderm), and an elongated notochordal band or rod of cells 

 extending backward between the hypoblast cells (fig. 209C) to Hensen's node, 

 where it unites with the hypoblast posteriorly (fig. 209D). Unlike the condi- 

 tion in the chick, the notochordal rod, other than in the pre-chordal plate 

 area, is exposed to the archenteric space below (fig. 209C). It simulates 

 strongly that of the reptilian blastoderm as gastrulation draws to a close. 



In the meantime, mesodermal cells from the primitive streak migrate for- 

 ward between the hypoblast and epiblast along either side of the notochord 

 in the form of two wing-like areas (figs. 208H, I; 209C). Other meso- 

 dermal cells migrate posteriad and laterad. Consequently, one is able to dis- 

 tinguish two main groups of mesodermal cells: 



( 1 ) formative or embryonic mesoderm, which remains within the confines 

 of the embryonic or germinal disc and 



(2) distally placed non-formative or extra-embryonic mesoderm. 



The former will give origin to the mesoderm, of the embryonic body, while 

 from the latter arises the mesoderm of the extra-embryonic tissues. 



In conclusion, therefore, we may assume that, during gastrulation in the 



