404 DEVELOPMENT OF FROG AND OTHER VERTEBRATA 



spermatozoon then moves toward the nucleus of the egg (Fig. 213). 

 Before the entrance of the spermatozoon the egg is radially S3^m- 

 metrical around its polar axis, but during the fertilization a 

 bilateral symmetry is established. There are further changes in 

 the egg as the male and female nuclei approach one another. As 

 seen in the plane of section shown by the figure, the cytoplasm 

 becomes differentiated by segregation of its material into several 

 regions, with the result that, even before the division of the zygote 

 begins, it is possible to recognize the portions of the egg from 

 which the ectoderm, endoderm, mesoderm, nervous system, and 

 notochord will arise in the subsequent development. Although 

 this point is difficult of determination, it also appears that the 

 plane of bilateral symmetry in the adult animal coincides approx- 

 imately with the bilaterahty that is estabHshed by the entrance 

 point of the spermatozoon and by the areas into which the cyto- 

 plasm comes to be differentiated. The anterior and posterior, 

 dorsal and ventral regions, and other parts of the adult body may 

 be traced through the susequent stages. In this manner an organ- 

 ization, which foreshadows that of the adult animal, is developed 

 within the cytoplasm of the zygote in advance of any cell division. 



Cleavage, — The cell division, or cleavage, by which the zygote 

 is transformed into a many-celled individual, begins soon after the 

 union of the male and female nuclei, by a division of the cell into 

 equal halves, in a plane that passes through the polar axis (Fig. 

 213 D). The external furrow of this two-cell stage, which begins 

 at the animal pole and extends downward on either side until it 

 encircles the egg, is the final stage by which the cytoplasm of the 

 cell is divided following the division of the nucleus. In most 

 instances the plane of this first cleavage seems to coincide with 

 the plane of symmetry already estabhshed by the areas of cyto- 

 plasmic differentiation that have been described. Hence the right 

 and left halves of the future embryo are usually represented by 

 right and left cells in the two-cell stage. 



The second cleavage, by which the four-cell stage is produced, is 

 at right angles to the first along the polar axis (Fig. 214). The 

 third cleavage, which forms the eight-cell stage, is at right angles 

 to the previous ones in a plane parallel to, but somewhat above 

 the equator. It thus separates four smaller, darkly pigmented 

 cells toward the animal pole from four larger, lighter-colored cells 

 in the vegetative hemisphere. From this time onward, the 



