352 PROCEEDINGS OF THE ACADEMY OF [April, 



tips in size, which are directed toward the vegetative pole (PI. XXIII, 

 figs. 21, 22, 23, PI. XXIV, fig. 24). 



The second quartet at this time consists of four similar groups of 

 cells, each group consisting of two large cells, 2a^^-2d^^ and 2a^^-2d2', 

 lying together, with the smaller cells above and below. The two large 

 cells in all four quadrants, 2a*2-2d^^ 2ar^-2cV\ next divide almost 

 simultaneously. The direction of cleavage of the right upper cells 

 (2a^2_2di2) is dexiotropic, and of the resulting cells the upper (2a^-^- 

 2d*2') are slightly larger than the lower (2a}'^'^-2cV^^) , the divisions being 

 identical in all four quadrants. Synchronously with these divisions 

 cleavage spindles appear in the other large cells of the second quartet 

 {2a?^-2(P^). Of the resulting cells the lower are much the smaller. 

 In direction the cleavages are probably all Iseotropic and therefore 

 non-alternating, though in C and D quadrants the spindles are almost 

 meridional in position, and the cleavages horizontal. Figures 28, 29, 

 30, 31 and 32 show these divisions in the different quadrants. 



The lack of alternation found in the above instance may be explained 

 as the direct result of the relative sizes of the foregoing derivatives of 

 the second quartet and the positions in which they lie. By an exami- 

 nation of fig. 30 it will be seen that should the two large cells, 2c^2 and 

 2c^\ have divided in the same direction a diagonal row of cells would 

 have been the result, with great pressure against one another and upon 

 the cells in the first and third quartets at the ends of the row. Lack 

 of alternation in direction of cleavage in one of the cells would relieve 

 this pressure, and this is the actual condition found. Such an expla- 

 nation appears to fit this individual case of non-alternation, but no 

 generalization may be made, as in many other instances the cleavage 

 of blastomeres appears to follow no rules of mutual pressure and can 

 be explained on no grounds so simple. 



Division again occurs in this quartet at a stage of alDOut eighty cells 

 and great variation in time is marked in their occurrence. 



The following table shows the average sequence observed in the 

 different quadrants, though any one egg may show marked variation 

 from the tabulated result: 



