336 EDWIN G. CONKLIN 



normal positions; in figure 65 one pole of the spindle has been 

 displaced more than the other one and the spindle is therefore 

 oblique to the chief axis of the egg; in figures 66 et seq. the 

 spindle was moved into the chief axis and the resulting cleavage 

 is nearly equatorial in position. 



Equatorial cleavages are of particular interest since they afford 

 an opportunity of studying critically the polarity of the egg and 

 the potency'' of its different parts. When the first (or second) 

 cleavage is changed from a meridional to an equatorial one there 

 is a totally new collocation of nuclei and cytoplasmic substances 

 with respect to the original axis and poles of the egg. Never- 

 theless, the original polarity is preserved almost unchanged or 

 if modified at all is subsequently restored. In the cell below 

 the equator, cytoplasm, centrosphere and nucleus move during 

 telokinesis to the free surface and as near as possible to the 

 animal pole; in the cell above the equator a similar movement is 

 limited apparently by some attraction on the part of the proto- 

 plasm of the lower cell so that cytoplasm, centrosphere and 

 nucleus come to occupy a position more or less intermediate be- 

 tween the lower cell and the animal pole (figs. 67 to 73). 



If an egg in which the first cleavage was equatorial is fjeed 

 from pressure and permitted to develop, the second cleavage 

 will be meridional (figs. 71, 72, 75 to 78) and each of the four 

 macromeres thus formed will give rise to three sets of micro- 

 meres (ectomeres) precisely as in a normal egg, except that the 

 micromeres of the cells below the equator are not able to reach 

 the animal pole though they move as far as possible in that 

 direction (figs. 77 to 81). Figure 123 represents an egg in 

 which the second cleavage was equatorial in position. The four 

 macromeres have given off the first set of micromeres (la-Id) 

 and are in process of forming the second set {2a-2d) ; the posi- 

 tions of the spindles and of the cleavage cells are somewhat 

 abnormal in most of the cells. Whatever the factors are 

 which in normal eggs determine the equality of the first two 

 cleavages and the inequality of succeeding cleavages, as well as 

 the polarity of egg and cleavage cells, these factors are still 

 present in centrifuged eggs in which the first or second cleavage 



