i3« 



BIOLOGICAL LECTURES. 



in which case the same final arrangement would result. This 

 is an interesting phenomenon, since it shows that the re- 

 arrangement of cells after cleavage is completed may take 

 place independently of the law of minimal contact surfaces, 

 and that, though this law may suffice to account for the re- 

 arrangement in some cases, yet in others some other force is 

 the efficient cause. 



At the next cleavage two of the nuclei divide at right angles 

 to the plane of the preceding division, as does also a third one, 

 though its division plane is also at right angles to those of the 

 other two. So far the cleavage follows the rule of rectangular 

 division, but the fourth nucleus has its spindle arranged prac- 

 tically at an angle of 45° to those of all the other nuclei. The 

 result of this division is shown in Fig. 7, in which one finds at 



one extremity of the somewhat 

 oval egg a circle of four nuclei 

 A, A', B, B', which are derived 

 from the nuclei A and B of Fig. 6, 

 while at the other extremity is a 

 single nucleus D, and between it 

 and the circle of four is a circle of 

 three nuclei C, C, and B'. What 

 determines the peculiar direction 

 of the spindle of the nucleus I) of 

 the preceding stage ? It is not in 

 accordance with the law of rec- 

 tangular division, nor can it be 

 determined by the principle of least resistance, for if this prin- 

 ciple acts, a similar arrangement at both extremities of the 

 ovum should be found. We are not, I believe, yet in a position 

 to determine the ultimate cause of this division, but perhaps 

 some clew to its significance may be obtained by carrying the 

 development on a stage or two. 



Let us examine the stage in which thirty-two nuclei are 

 present. At this stage several marked peculiarities are to be 

 seen. In the first place there is a striking differentiation of 

 the protoplasmic masses which surround the nuclei, an appear- 

 ance such as is represented in Fig. 8 being produced. At one 



Fig. 7. 



