CLEAVAGE AND DEVELOPMENTAL PATTERN 589 



Gradient pattern and morphological pattern in ciliate protozoa are lim- 

 ited to the ectoplasm, and results of centrifuging eggs indicate that de- 

 velopmental pattern may be continuously present only in the cortex, 

 though perhaps extending to deeper cytoplasm in the oocyte. If centri- 

 fuging involves the cortex or cytoplasmic areas closely associated with it, 

 such as the polar plasms of Tubifex and Clepsine, developmental pattern 

 may be altered. It appears, however, that ultracentrifuging may alter 

 ventrodorsality and even polarity in some eggs with spiral cleavage, but 

 whether the alteration results from effect on the cortex or on the deeper 

 cytoplasm remains to be determined. 



If a physiological gradient pattern involving metabolism is continu- 

 ously present in the cortical cytoplasm, the hypothesis of an elastic or 

 contractile network, which is merely stretched or distorted by displace- 

 ment of yolk and other substances in centrifuging and brings the particles 

 back into place after the force ceases to act, seems scarcely necessary. 

 Assuming presence of a gradient pattern, the original distribution of cyto- 

 plasmic granules or inclusions and its changes in maturation, fertilization, 

 and cleavage are undoubtedly related in some way to this pattern. It 

 appears probable that differences in condition at different gradient-levels, 

 perhaps extending to the deeper cytoplasm, may bring about the return to 

 normal structure and distribution. Electric-potential differences and dif- 

 ferences in surface tension and in colloidal dispersion are possible factors. 



ALTERATION OF CLEAVAGE BY OTHER FACTORS IN RELATION 

 TO DEVELOPMENT 



It is a well-known fact that mitotic spindles tend to orient vertical to 

 the direction of mechanical pressure and in the greatest cell diameter, 

 though other factors may prevent such orientation in many cases. The 

 stratification resulting from centrifuging is apparently very similar to 

 mechanical pressure in its effect on orientation of spindles in many cells. 

 Alterations of cleavage pattern and dislocation of blastomeres have been 

 produced in various eggs by pressure and in some by other means, such as 

 violent pipetting, shaking, and exposure to calcium-free sea water. 



Cells of the early blastula of a medusa, Aegineta, after extreme dislo- 

 cation by pipetting, tend to reconstitute a spherical blastula (Fig. 184) 

 and to give rise to normal individuals (Maas, 1901). The apparent polar 

 difference in size of blastomeres in Figure 184, B and C, suggests that a 

 more or less normal arrangement may be regained. If a gradient pattern 

 is present in the blastula, as in other coelenterate blastulae (pp. 96, 167), 



