44 



TEXT-BOOK OF EMBRYOLOGY. 



(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 proto- 

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



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 portion of 

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

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

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

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

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

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

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

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

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

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

 cells and the same number of lower yolk cells, this uniformity of number per- 



