DEVELOPMENT OF THE LEOPARD FROG 407 



separate each cell of the eight-cell stage into halves. Since these 

 planes of the fourth cleavage begin at the animal pole and cut 

 through the four smaller cells before they extend to those of the 

 vegetative area, a twelve-cell stage is produced before the sixteen- 

 cell stage is formed. Two fifth cleavage planes next appear, one 

 above and one below the third furrow, dividing each cell of the 

 sixteen-cell stage and forming a thirty-two-cell stage. In theory, at 

 least, these are parallel to the equator and therefore at right angles 

 to those that preceded them, but in fact the pattern of the mass of 

 cells has by this time become so irregular that cleavage planes 

 are followed with difficulty. 



The feature which is apparent throughout is that the cell 

 activities progress more rapidly at the animal pole, as shown by 

 the way the first and second cleavage planes begin to appear in the 

 animal hemisphere and extend to the vegetative pole, and by the 

 more rapid division of all the cells of the animal hemisphere during 

 the later cleavage stages. As a result, the cell outhnes at the 

 vegetative pole remain visible long after those of the animal pole 

 have become indistinguishable to the eye. This difference in the 

 rate of activity in the two hemispheres is an expression of the 

 internal relationships, whereby the nucleus of the egg becomes 

 eccentric and yolk accunmlates in the vegetative hemisphere 

 before the egg leaves the ovary {cf. Fig. 210). Since it contains 

 relatively httle active protoplasm, the vegetative half of the 

 egg does not develop so rapidly as the animal region. The 

 appearance of the third cleavage furrow somewhat above the 

 equator is a part of this relationship, although its immediate cause 

 is the position of the original cleavage nucleus, and thus of the 

 nuclei of the four-cell stage, in the upper hemisphere. From the 

 thirty-two-cell stage onward the cleavage proceeds rapidly by 

 further cell divisions, some of which are vertical while others are 

 at right angles to the surface, while the internal cavity, which 

 begins to appear even in the eight-cell stage, enlarges rapidly. 

 The stage thus developed is called the blastula. 



The Blastula. — The blastula stage, which occurs in the develop- 

 ment of many animals (cf. Fig. 148 E, p. 307, and Fig. 209 C), is a 

 spherical mass of cells enclosing an internal segmentation cavity, 

 or blastocoele. In the frog, the thirty-two-cell stage (Fig. 213 F) 

 may be called an early blastula, since the blastocoele has already 

 made its appearance by a separation of the cells at their inner ends, 



