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EMBRYOLOGY 



Fig. 57. A four-cell and an eight-cell salamander egg. Left: The intersection 

 of the first and second cleavage furrows at the animal pole. Compare with Figure 

 55, which shows the four-cell stage with both the animal and vegetal hemispheres 

 in view. Right: An eight-cell stage with four small cells on top and four large 

 cells underneath. (Courtesy Professor ]ohn A. Moore, Columbia University.) 



The third cleavage gives rise to eight cells and is horizontal — that is, it 

 follows a line parallel to the equator. After this cleavage the egg is composed 

 of four animal cells and four vegetal cells, with the vegetal cells distinctly 

 larger (Fig. 55). The third cleavage is, therefore, holoblastic, unequal, and 

 horizontal. An eight-cell salamander egg showing the four relatively small 

 cells in the animal hemisphere is shown in Figure 57. These four cells are in 

 direct contact with four larger cells in the vegetal hemisphere. 



The fourth cleavage of the frog egg passes through the animal-vegetal 

 axis and follows the meridians of the egg. The result is a cluster of eight 

 relatively small cells in the animal hemisphere and eight relatively large cells 

 in the vegetal hemisphere. In Figure 55, stage 6, the fourth cleavage passed 

 through the animal cells but is only part way through the vegetal cells. 



Late cleavage through gastrulation 



The next series of changes (Fig. 58) takes us from the 6th hour to the 

 28th hour of development. From 6 through 16 hours cleavage continues and 

 the cells at the animal pole become very small; at 12 and 16 hours, therefore, 

 the individual cells are no longer shown. The vegetal cells are relatively 

 larger and are easily seen at 16 hours. Photographs of salamander eggs at 



