42 TEXT-BOOK OF EMBRYOLOGY. 



of Amphioxus thus presents four upper smaller cells and four lower larger 

 cells (Fig. 26). 



The difference in size between the four upper and the four lower blastomeres 

 in Amphioxus finds probable explanation in the distribution of yolk within the 

 egg and the first four blastomeres. The yolk is greater in amount at the lower 

 pole of the cell, thus leaving the greater amount of protoplasm at the upper 

 pole. The nucleus tends to occupy the center of the protoplasmic mass and 

 consequently is nearer the upper pole. Therefore when the spindle forms 

 about the nucleus, the plane bisecting the spindle at right angles will be nearer 

 the upper pole of the cell. This plane corresponding to the division plane 

 of mitosis, the two resulting cells will be unequal in size, the smaller one 



Micromeres 



Segmentation 

 cavity 



Macromeres 



FIG. 26. Cleavage of the ovum of Amphioxus. Hatschek, Bonnet. 

 1-5, Lateral views of segmenting cells; 6, section of blastula. 



lying above and the larger below. Thus is shown one of the effects of yolk 

 distribution. 



(B) UNEQUAL. A good example of this form of cleavage is found in the 

 common frog's egg (Fig. 27). This egg while containing little yolk when com- 

 pared with such eggs as those of the fowl, contains much more yolk than does 

 the egg of Synapta or of Amphioxus. The frog's egg being a telolecithal egg, 

 the yolk is gathered at one pole, enabling a distinct differentiation to be made 

 between the upper darker protoplasmic or animal pole, and the lower lighter 

 vegetative pole (Fig. 6). The cleavage is complete but the cells which develop 

 at the yolk pole are much larger than those which develop at the protoplasmic 

 pole. The first and second cleavage planes are as in Synapta and Amphioxus, 

 vertical and at right angles to each other. Each of the four cells which result 

 from the second cleavage in the frog consists of a small upper darker protoplas- 

 mic pole and of a larger lower lighter yolk pole (Fig. 27, A). The nuclear 



