486 



BERTRAM G. SMITH 



\III. (lASTRULATIOX AXD EARLY FORMATION' OF THE EMBRYO 



A. Description hij nUiges 



Stage 11: (figs. 113 to 137 and 220 to 222). This stage extends 

 from the time of the first appearance of the blastopore as a short 

 horizontal groove until its ends meet to form a complete circle. 

 In eggs kept in their natural environment, gastrulation begins 

 about seven days after fertilization and two days after the begin- 

 ning of Stage 10. 



Figs. 113 and 114 Lower hemispheres of two eggs of Cryptobranchus alle- 

 gheniensis in an early gastrida stage, showing cleavage furrows. The vertical 

 axis as determined by gravity lies at the center of each figure; the vegetal pole, 

 at the intersection of the first two cleavage furrows, is about 7 degrees above the 

 vertical pole. \n figure 113 the first cleavage furrow lies approximately in the 

 median plane of the gastruhi; in figure 114 it is at right angles to this plane. Cam- 

 era drawings, finished under the binocuhir, from jjreserved material. X 8. 



The blastopore is first distinguished as a shallow irregular and 

 broken horizontal groove two or three millimeters in length, 

 lying about 15 degrees below the equator. It occurs at the upper 

 limit of transitional cells between micromeres and macromeres, 

 and its immediate site is distinguished by a rather abrupt demar- 

 cation between micromeres and distinctly larger transitional 

 cells. The groove is started, not by a lining-up of cells and the 

 union of cleavage furrows, as described by Eycleshymer ('95) 

 for Amblystoma, but by the sinking-in of groups of entire cells 



