GASTRULATION IN VARIOUS CHORDATA 441 



to the inside on either side of the median, notochordal mass and above 

 the forming, secondary hypoblast. 



b) Epiboly. The germ-ring tissue and the outer areas of the presumptive 

 epidermal cells gradually grow around the yolk mass and converge toward the 

 caudal end of the developing embryo. Associated with this migration of cells 

 is the anterior-posterior extension of the presumptive neural plate material 

 to form an elongated, thickened, median ridge. 



4) Developmental Potencies of the Germ Ring of Teleost Fishes. The 

 germ ring or thickened, marginal area of the teleost late blastula and early 

 gastrula has interested embryologists for many years. It was observed in 

 Chapter 8 that various regions of the marginal area of the blastoderm of the 

 teleost fish have a tendency to form embryos. Luther ('36), working on the 

 trout (Salmo), found that all sectors of the blastula were able to differentiate 

 all types of tissue, i.e., they proved to be totipotent. However, in the early 

 gastrula, only the sector forming the embryonic shield and the areas immedi- 

 ately adjacent to it were able to express totipotency. As gastrulation progresses, 

 this limitation becomes more marked. In other words, a generalized potency 

 around the germ ring, present during blastulation, becomes restricted when 

 the embryonic shield of the gastrula comes into prominence. The evidence set 

 forth in the previous chapter indicates that the possibility for twinning in the 

 trout becomes less and less as the gastrular condition nears. The restriction 

 of potency thus becomes a function of a developmental sequence. 



In the case of Fundidus, Oppenheimer ('38) found that various areas of 

 the germ ring, taken from regions 90 degrees or 180 degrees away from the 

 dorsal blastoporal lip, were able to differentiate many different embryonic 

 structures // transplanted into the embryonic shield area. Oppenheimer con- 

 cludes that: "Since under certain conditions the germ-ring can express poten- 

 cies for the differentiation of many embryonic organs, it is concluded that its 

 normal role is limited to the formation of mesoderm by the inhibiting action of 

 the dorsal lip." The results obtained by Luther serve to support this conclusion. 



c. Gastrulation in Elasmobranch Fishes 

 In figure 179B the presumptive major organ-forming areas of the blasto- 

 derm of the shark, Scyllium canicnla, are delineated. The arrows indicate the 

 general directions of cell migration during gastrulation. In figure 212A-G are 

 shown surface views of the dorsal-lip area of different stages of blastodermic 

 development in this species, while figure 213A-G presents median, sagittal 

 sections of these blastoderms during inward migration of the presumptive 

 organ-forming cells. It is to be observed that the dorsal-lip region of the 

 blastoderm is the focal area over which the cells involute and migrate to the 

 inside. 



