120 



FISHERY BULLETIN OF THE FISH AND WILDLIFE, SERVICE 



Blastocoel: Progressively obscured by opacity 

 of animal hemisphere. Decreases in volume. 



Blastopore: Forms as wide arched slit. Hooded 

 as neural plate begins to form. Apparently 

 migrates. 



The translucency of the animal hemisphere of 

 stage 8 changes to transparency after the appearance 

 of the blastopore. Through the transparent animal 

 hemisphere the underhdng chorda-mesoderm is 

 visible as it undergoes its morphogenetic move- 

 ments. During this period the volume of the 

 blastocoel decreases when the chorda-mesoderm 

 reaches a position two-thirds the distance across 

 the animal hemisphere. The most advanced 

 portion of the chorda-mesoderm is in the mid- 

 sagittal plane; the material in the parasagittal 

 planes lags behind these more advanced cells. 

 As this m.aterial progresses beneath the animal 

 hemisphere, the dorsal lip of the blastopore pro- 

 gressively increases in thickness and begins an 

 apparent migration from its original position at 

 or near the overlap of animal and vegetal cells 

 one-third the distance from the center of the animal 

 hemisphere, toward the center of the animal hemi- 

 sphere. As the chorda-mesoderm moves to its 

 anteromost position in the embryo, the trans- 

 parency of the animal hemisphere decreases, and 

 the decrease in the volume of the embryo continues. 



The typically circular blastopore of later stages 

 is formed by progression of the animal cells in their 

 epibolic movements to produce a changing pattern 

 to the dorsal lip and, necessarily, the blastopore. 

 At the time of the blastopore's inception the 

 advancing margins of epibolizing cells beneath the 

 blastopore approach each other laterally as a 

 wide-open V with the apex at the center of the 

 dorsal lip of the blastopore. As gastrulation pro- 

 gresses, these V-arranged margins close toward 

 each other until only the vegetal cells below the 

 blastopore remain uncovered. These vegetal 

 cells are covered by epibolizing cells which move 

 upward along the mid-sagittal plane (fig. 12). 

 As the chorda-mesoderm advances farther into 

 the anterior portion of the embryo, the blastopore 

 begins an apparent migration along the mid-sagit- 

 tal plane from its original position to one located 

 at the posterior limit of the mid-sagittal plane in 

 stage 10. Histological comparisons between stages 

 8 and 9 indicate that this apparent movement re- 

 sulted from reduction in the size of the blastocoel. 



At this time the dorsal region of the entire embryo 

 begins to flatten and to thicken from the dorsal lip 

 of the blastopore to the anterior region. 



Stage 9 is marked by the appearance of the 

 dorsal lip of the blastopore, a flat crescentic- 

 shaped furrow within the overlapping line of the 

 animal cells epibolizing over the vegetal cells. 

 Recognition of the stage depends upon locating 

 the blastopore half-way beneath the embryo; the 

 observer must rotate the embryo to find it. 



The end-point of stage 9 is reached when the 

 flattening process reaches the anterior end of the 

 embryo. 



Stage 10: Neural plate and groove (fig. 13) days 4-5 



Size: 1.1 to 1.3 millimeters. 



Blastopore: Triangular to ovoid. Reaches its 

 dorsalmost point. 



Neural tissue: Neural plate forms and thickens. 

 Groove and folds form. 



Figure 13. — Lateral view of stage 10, neural plate. 



Stage 10 begins when the flattening of the dorsal 

 ectoderm mentioned in stage 9 has reached the 

 anterior extremity of the embryo (fig. 14). At the 

 same time, the nearly completed blastopore is 

 near or at the dorsalmost point of the posterior area 

 of the mid-sagittal plane of the embryo (fig. 15). 

 Further thickening of the depth of the flattened 

 area follows almost immediately after the flat- 

 tening of the dorsal ectoderm extends from the 

 blastopore to the anterior region. These animal cells 

 are no longer transparent. The ovoid to triangular 



