GASTRULATION IN VARIOUS CHORDATA 431 



(1) While the above activities take place, the area pellucida becomes 

 elongated posteriorly. The entire pellucid area thus becomes piriform, 

 i.e., pear-shaped (figs. 202F-I; 203C). 



(m) This change in shape of the pellucid area is associated primarily with 

 the activities involved in epiboly which accompany the embolic activi- 

 ties observed above. Epiboly brings about the elongation of the pre- 

 sumptive neural crescent, converting it into an elongated band of cells. 

 It also effects the expansion and antero-posterior extension of the 

 overlying presumptive, neural plate and epidermal cells. The latter 

 behavior is intimately associated with the antero-posterior extension 

 of the notochordal and mesodermal cellular areas mentioned in (j) 

 and (k) on pp. 427, 428. 



(n) Most of the gastrulative processes in the chick are completed at about 

 20 to 22 hours after incubation starts. At this time the blastoderm is 

 in the head-process stage. The so-called head process or "notochordal 

 process" represents the rudimentary notochord which projects forward 

 from the primitive streak. (See (j) on p. 427.) At this time the various, 

 specific, organ-forming areas appear to be well established (figs. 2021; 

 205A-E). (See Rawles, '36; Rudnick, '44.) From this time on the 

 primitive streak regresses caudally, as the embryo and embryonic tis- 

 sues develop in front of it. The caudal regression of the streak is shown 

 in figure 206. Spratt ('47) concludes that as the streak regresses, it 

 becomes shortened by transformation of its caudal end into both em- 

 bryonic and extra-embryonic ectoderm and mesoderm. Finally, the 

 anterior end of the streak, that is, the primitive knot or Hensen's node 

 together with possibly some condensation of adjacent streak tissue 

 (Rudnick, '44), forms the end bud. The latter, according to Homdahl 

 ('26) gives origin to the posterior portion of the embryo caudal to 

 somite 27 and to the tail. The remains of the end bud come to a final 

 resting place at the end of the tail. 



5. Gastrulation in Mammals 

 a. Orientation 



In the mammals, the formative area of the blastocyst (blastula) is located 

 at one pole and is known as the embryonic or germ disc. It consists of a lower 

 hypoblast and an upper epiblast. This embryonic disc is connected to the 

 non-formative or trophoblast cells around its edges (figs. 176, 177, 178). 

 In some species the embryonic disc is superficial and uncovered by trophoblast 

 cells (pig, cat, rabbit, opossum), while in others, it is sequestered beneath a 

 covering of trophoblast (human, monkey, rat). (See figs. 177, 178.) 



