THE EARLY DEVELOPMENT OF THE FROG 83 



toward the center of the egg, approach and meet in the usual 

 manner. The path of the egg nucleus is not marked by any 

 pigment, nor is it accompanied by any radiations such as were 

 connected with it during its maturation. The sperm centro- 

 some-and centrosphere divide and form the poles of a small 

 but typical cleavage figure which is not located near the center 

 of the egg, but always toward the animal pole. The position 

 of the first cleavage spindle is not entirely undirected, but 

 before we can discuss this point we must consider some facts 

 regarding the structure of the egg itself after fertilization. 



2. The Symmetry of the Egg 



Before fertilization the egg has a well-marked polarity and is 

 radially symmetrical about its chief axis (Fig. 29, A). This 

 form of radial symmetry (not spherical) has been termed 

 " rotatory/' i.e., radially symmetrical in any plane at right 

 angles to the chief axis. The vegetal pole contains a large 

 proportion of yolk, while the animal pole is relatively free 

 from yolk and is covered externally by a thin but dense coating 

 of brown or black pigment; moreover, the nuclear structures 

 are in the animal pole (Fig. 23, C). The specific gravity of 

 the lower pole is the greater, on account of the heavy yolk con- 

 tained in it, and therefore the pigmented animal pole is turned 

 upward when the egg is free to rotate. This rotation, however, 

 is not usually completed for some minutes after the spermato- 

 zoon has entered and the egg membranes are somewhat freed 

 from its surface. 



But this radial or rotatory symmetry is not retained after the 

 entrance of the sperm, for this affords the stimulus which leads 

 to a rearrangement of the substance of the egg, accompanied 

 or followed by the rapid development of a bilateral symmetry 

 in the egg, with which that of the embryo tends strongly to 

 coincide. The factors determining the position of this new 

 plane of bilateral symmetry are really three-fold, one primary 

 and two secondary. The primary factor is the polar and 

 rotatory symmetry of the unimpregnated egg; the plane of 



