328 CLEAVAGE (SEGMENTATION) AND BLASTULATION 



period. This subject was investigated in the amphibian blastula by Holtfreter, 

 '39. According to this investigator, blastomeres, when isolated by mechanical 

 means, appear to wander aimlessly about. When contact is made with other 

 blastomeres during this wandering process the cells stick or adhere together. 

 As a result, the mass of adhering cells gradually is formed which becomes 

 rounded into a ball-shaped structure. The results of this work suggest that 

 the force which draws the cells together is one of thigmotaxis or contact 

 affinity, aided by a surface stickiness of the cells. This force only becomes 

 influential when an isolated cell has made contact with another cell or cells. 



On the other hand, the early blastomeres of the cleaving mammalian egg 

 are evidently held together also by the binding influence of the egg membrane 

 or zona pellucida. An adhering influence is not prominent until later cleavage 

 stages. 



However, one must not be too ready to espouse a single, mechanical factor 

 as the main binding force which causes the blastomeres to adhere together, 

 to move in relation to each other, and to form a definite configuration. Factors 

 tending toward organization are at work during early and late cleavage as 

 well as in subsequent development. Relative to these matters, it is well to 

 cogitate upon the statement of Whitman (1893). "Comparative embryology 

 reminds us at every turn that the organism dominates cell-formation, using 

 for the same purpose one, several, or many cells, massing its material and 

 directing its movements, and shaping its organs, as if cells did not exist, or 

 as if they existed only in complete subordination to its will" (p. 653). 



D. Progressive Cytoplasmic Inequality and Nuclear Equality of the 

 Cleavage Blastomeres 



1. Cytoplasmic Inequality of the Early Blastomeres 



In harmony with the differences in the location and activities of the various 

 blastomeres of the cleaving egg, it is apparent that a difference exists in the 

 ooplasmic substance within the various cells in many species. In the frog, 

 for example, the quantity of yolk substance present in the cells of the yolk 

 pole is much greater than that of the animal pole. Similarly in the four-cell 

 stage the substance of the gray crescent is located in two of the blastomeres, 

 while the other two blastomeres have little or none of this substance. Two 

 of these four cells, therefore, are qualitatively different from the other two. 

 In the ascidian, Styela partita, the presence of the yellow crescent, yolk sub- 

 stance, and gray crescent materials demonstrates that in the four- or eight- 

 cell stages there are qualitative differences in the ooplasmic substances which 

 enter into the composition of the respective blastomeres (Conklin, '05, a 

 and b). Similar conditions may be demonstrated for Amphioxus although 

 pigmented materials are not present in the egg (fig. 167). (See Conklin, '32, 

 '33.) As cleavage continues in the eggs of Styela and Amphioxus, a progres- 



