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CLEAVAGE (SEGMENTATION) AND BLASTULATION 



Fig. 149. Early cleavage in Ainbystoma inuculatum (punctatum). (After Eycleshymer, 

 J. Morphol., 10, and eggs in the laboratory.) (A, B) First cleavage furrow, meridional 

 plane. (C) Second cleavage furrow at right angles to first furrow, meridional plane. 



(D) Third cleavage furrow, latitudinal, forming four micromeres and four macromeres. 



(E) Fourth cleavage furrow; mixture of meridional and vertical planes of cleavage. (F) 

 Fifth cleavage furrows; mixture of latitudinal and vertical planes of cleavage. Observe 

 equatorial plane cutting the large macromeres. (G-I) Later cleavage stages. 



meroblastic cleavage. For example, the fourth set of cleavages may be a 

 mixture of vertical and meridional furrows, as shown in figure 149E. The fifth 

 cleavages are a mixture of horizontal (i.e., latitudinal and equatorial, fig. 

 149F), vertical and meridional furrows. The sixth set of cleavages is made 

 up of vertical and horizontal cleavage planes of considerable variableness (fig. 

 149G). From this time on cleavage becomes most variable, with the animal 

 pole micromeres dividing much more rapidly than the yolk-laden macromeres 

 at the vegetal pole (figs. 149H, I). 



The blastocoel makes its appearance at the eight-cell stage and appears 

 as a small space between the micromeres and the macromeres, the latter 

 forming the floor of the blastocoelic space. At the late blastula stage, the 

 blastocoel is roofed over by the smaller micromeres, and floored by the yolk- 



