44 TEXT-BOOK OF EMBRYOLOGY. 



(B) UNEQUAL. A goocl example of this form of cleavage is found ii 

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

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

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

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

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

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

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

 pole. The first and second cleavage planes are as in Synapta and Amphi< 

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

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

 plasmic pole and of a larger lower lighter yolk pole (Fig. 27, A). 



Micromeres 



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. 



nuclear elements lying, as they always do, within the protoplasmic porti 

 the cell, determine the next cleavage plane which is horizontal and lies n 

 the protoplasmic ends of the cells. The result is that the third cleavage 

 rise to eight cells, four of which are small protoplasmic cells lying abov 

 line of cleavage, while the other four are large yolk-containing cells whic 

 below the line of cleavage (Fig. 27, A). This distinction between protoph 

 cells and yolk cells not only persists but tends to become more and more rm 

 as segmentation proceeds, and it soon becomes evident that the cells unen 

 bered by yolk have a tendency to segment more rapidly than do their 

 laden brethren (Fig. 27, C, D, E, F and G). Thus, while the fourth c 

 age is vertical in both types of cells, giving rise to eight upper protoph 

 cells and the same number of lower yolk cells, this uniformity of numbei 



