GENERAL ZOOLOGY 



noticeable activity is the overgrowth of the cells of the vegetal hemisphere 

 by those of the animal hemisphere, so that the pigmented area increases; 

 the non-pigmented area decreases in extent. As the spread of the pig- 

 mented cells occurs, the region of the gray crescent can still be identified, 

 and at the pigmented border of this region an inward movement of cells be- 

 gins. In other words, the cells derived from the gray crescent come to lie 

 inside the gastrula in the region of the dorsal lip of the blastopore and in what 

 is to be the mid-dorsal region of the embryo. The part of the germ ring from 

 which inturning first occurs is known thenceforth as the dorsal lip of the 

 blastopore, or the opening into the gastrocoel. Soon the inward shift of cells, 

 or involution, occurs along the entire margin of the germ ring, which thus 

 becomes the lips of the blastopore. In amphibia the blastopore is plugged 

 with cells of the vegetal hemisphere which have not shifted their position 

 (Fig. 5.15/s). The overgrowth of the yolk-laden cells by pigmented cells is 

 known as epiboly and continues, with the resulting decrease in the circum- 

 ference of the germ ring and in the area of the yolk plug, until the yolk plug 

 is covered and the blastopore is a minute opening. Internally, the gastrula 

 cavity increases greatly in extent as a result of invagination and involution, 

 while the blastula cavity decreases in size as the gastrula cavity expands. As 

 gastrulation progresses, it is possible to distinguish dorsal and ventral sur- 

 faces, as well as anterior and posterior ends, because of their subsequent 

 development. Furthermore, the expansion of the gastrula cavity in the dorsal 

 half of the gastrula and the obliteration of the blastula cavity in the animal 

 hemisphere result in a rotation of somewhat more than 90 degrees in the posi- 

 tion of the individual within its jelly envelopes. Instead of the animal 

 hemisphere, the dorsal half now floats uppermost. 



As the blastopore closes in amphibia, the shift of cells from an external to 

 an internal position is completed. The cells which remain on the surface are 

 the ectoderm; those which have moved in and now hne the gastrocoel dorsally 

 and laterally are the presumptive notochord and mesoderm. Farther down 

 along the sides of the gastrocoel the inwardly shifted cells are the true 

 endoderm. In the floor of the gastrula cavity these cells receive additions 

 from the yolk-laden cells by a process of delamlnatlon, or rearrangement, to 

 form a definite layer. Laterally, at the junction between the endoderm and 

 the presumptive mesoderm, a separation occurs between the two. Division 

 of cells in each region extends the layers. The two sheets of endoderm move 

 dorsally until they meet one another in the mid-dorsal line to form a con- 

 tinuous lining for the gastrula cavity. This changes the presumptive mesoderm 

 into a middle layer, the true mesoderm, lying between the ectoderm and 

 endoderm. Its free ventral margin on each side extends until the two sheets 

 meet midventrally. Dorsally, the mesoderm is continuous with a median 

 mass of cells derived by involution from the dorsal lip of the blastopore 

 (Fig. 5.15G). This association constitutes the chordamesoderm. Presently 

 the sheets of the mesoderm are no longer continuous with the mid-dorsal 

 cells, which become arranged as a longitudinal cord of cells known now as the 



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