256 Basic Structure of Vertebrates 



of the embryo; in the amphibian the eventual interior position of the 

 endoderm material is due mainly to the enclosing of the yolk-mass by 

 overgrowth (epiboly) carried out by the fold which was initiated by 

 invagination. In Amphioxus the endoderm goes inside; in the amphib- 

 ian it is put inside by being covered over. The essential process in 

 formation of the archenteron of Amphioxus is invagination. The am- 

 phibian archenteron results from interaction of three processes, invag- 

 ination, epiboly, and re-arrangement of yolk-cells. Quite clearly, the 

 difference is the necessary consequence of the presence of the great 

 mass of inert yolk in the amphibian blastula. 



In Reptiles and Birds. A reptilian or avian embryo, whose yolk- 

 mass may be millions of times that of Amphioxus, could hardly be 

 expected to carry out a process of gastrulation similar to that of 

 Amphioxus — if, indeed, anything comparable to gastrulation were to 

 be recognized at all. Yet the original single layer of the blastoderm, 

 formed by cleavage (Figs. 208, 209), must somehow give rise to addi- 

 tional layers. The fact is that the blastoderm does, at an early period, 

 become two-layered. The details of the mode of origin of the second 

 layer differ considerably in various members of the Sauropsida (reptiles 

 and birds). The significant fact is that the deeper layer (endoderm) 

 results, in part if not entirely, from an inward movement of blastoderm 

 cells at the median region of what proves to be the posterior edge of the 

 blastoderm (Fig. 212). This inward movement may consist in the for- 

 mation of a small pit, an actual invagination, from whose bottom cells 

 move forward and laterally underneath the original blastodermic layer 

 to become the endoderm. In other cases there is merely an inturning of 

 the midposterior edge of the blastoderm without formation of a com- 

 plete pocket or invagination. In either case the process is confined to 

 the midposterior region of the edge of the blastoderm. The endoderm, 

 thus initiated, rapidly spreads over the yolk-mass and under the orig- 

 inal layer, which is now identified as the ectoderm. The growth of the 

 endoderm may be augmented by cells which become detached from 

 the under surface of the outer layer. 



Fig. 212. Gastrulation in the pigeon. Section approximately median, showing 

 formation of endoderm by invagination at posterior edge of blastoderm. (A) 

 Archenteron; (B) blastocoele (cleavage cavity); (BP) blastopore; (EC) ectoderm; 

 (EN) endoderm; (V) vitelline membrane; (Y) yolk. (Magnified about 100 diam- 

 eters.) (After Patterson. Courtesy, Neal and Band: "Chordate Anatomy," 

 Philadelphia, The Blakiston Company.) 



