THE EARLY DEVELOPMENT OF THE FROG 91 



plane of the embryo, and the plane of the first cleavage furrow, 

 all to coincide, but all relations in the same axial plane are 

 possible among these and are actually found. 



3. Cleavage 



Cleavage of the frog's egg is total and unequal. The first 

 cleavage spindle lies in the direction of the greatest protoplasmic 

 extent, i.e., transversely to the egg axis, and in a position 

 determined by several different factors as described above (Fig. 

 27, C). The first cleavage furrow becomes visible on the surface 

 first at the animal pole, and gradually extends thence as a 

 narrow groove around a meridian of the egg to the vegetal pole; 

 it is completed about two and one-half hours after ensemination, 

 or much sooner if the temperature is raised slightly. While this 

 furrow is meridional, we have seen that it may or may not 

 divide the gray crescent symmetrically. Throughout cleav- 

 age the blastomeres remain in close contact so that they are 

 separated superficially by only shallow narrow grooves (Fig. 30), 

 and do not become distinctly rounded and separate elements 

 as in Amphioxus or in other eggs containing less yolk. 



The second cleavage appears about one hour after the first; 

 this is also meridional, at right angles to the first, .dividing 

 the egg into four adequal blastomeres. Succeeding divisions 

 appear about an hour apart. The third cleavage is the first 

 to divide the egg unequally; in the typical form of cleavage 

 this appears similarly in all four quadrants, and is latitudinal 

 or horizontal, i.e., at right angles to the first two (Fig. 30, A). 

 Although this cleavage plane divides the protoplasmic material 

 of the egg about equally, the accumulation of yolk in the lower 

 pole actually displaces this middle plane above the equator of 

 the egg, so that the cleavage furrow appears about sixty 

 degrees from the animal pole, and the egg as a whole is divided 

 unequally. Of the eight resulting cells, the four upper are 

 small and richer in protoplasm, while the four lower are large 

 and richer in yolk. This typical relation of the third cleavage 

 is by no means invariable. A considerable proportion, in 



