. Journal of morphology. distinguished as a shallow irregular andbroken horizontal groove two or three millimeters in length,lying about 15 degrees below the equator. It occurs at the upperlimit of transitional cells between micromeres and macromeres,and its immediate site is distinguished by a rather abrupt demar-cation between micromeres and distinctly larger transitionalcells. The groove is started, not by a lining-up of cells and theunion of cleavage furrows, as described by Eycleshymer (95)for Amblystoma, but by the sinking-in of groups of entire cells EMBRYOLOGY OF CRYPTOBRANCHUS 487 at
. Journal of morphology. distinguished as a shallow irregular andbroken horizontal groove two or three millimeters in length,lying about 15 degrees below the equator. It occurs at the upperlimit of transitional cells between micromeres and macromeres,and its immediate site is distinguished by a rather abrupt demar-cation between micromeres and distinctly larger transitionalcells. The groove is started, not by a lining-up of cells and theunion of cleavage furrows, as described by Eycleshymer (95)for Amblystoma, but by the sinking-in of groups of entire cells EMBRYOLOGY OF CRYPTOBRANCHUS 487 at intervals along a narrow zone several cells in width; hencefrom its very beginning the process is not a splitting-apart ofcells, but invagination. The groove soon becomes continuousand deepens by the inturning of cells along both margins. After the groove has reached a length of three millimeters ormore, the process of invagination becomes accompanied by oneof overgrowth or epiboly: the dorsal lip grows slowly down over. Fig. 115 Lower hemisphere of a gastrula of Cryptobranchus allegheniensis,in a slightly later stage than the preceding, showing the lining-up of the cells withinthe horns of the blastopore. Freehand drawing from a photograph of preservedmaterial. X 10. the cells transitional between micromeres and macromeres ( to 115). As shown in figure 115, the transitional cells justwithin the horns of the blastopore are elongated as if compressed;here the cells line up and lengthen out at right angles to a lineconnecting the horns of the blastopore. After the blastoporic groove has attained the form of a semi-circle (fig. 133), a zone of rather abrupt demarcation betweenmicromeres and transitional cells completes the circle begun bythe crescentic blastopore; this zone marks the site of the ventrallip of the blastopore. A little later, the blastoporic groove ex-tends rapidly along this line of demarcation, becoming an almost 488 BERTRAM G. SMITH perfect circle, and en
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Keywords: ., bookcentury1900, bookdecade1, booksubjectphysiology, bookyear1912
