CLEAVAGE 



67 



nature of the factors which in their turn determine the orientation of the 

 spindles is unknown, but there must be some sort of continuous change 

 proceeding within the cytoplasm which controls their development. 

 The occurrence of such changes is well shown by some experiments of 

 Horstadius (1939) on the echinoderm egg. He used eggs of the sea- 

 urchin Paracentrotus lividtis, which possesses a sub-equatorial band of 

 pigment, wliich makes it possible to recognise the orientation of the egg 

 even when the cleavage is abnormal. By treatment with hypotonic sea 

 water or by shaking, one can cause a delay in the appearance of the cleav- 

 age furrows, but the results show that the factors controlling the orienta- 

 tion of the spindles go through their usual changes at the normal rate. 

 Thus the first cleavage may be delayed until the spindle mechanism is 

 ready for the second cleavage, and the next until it is intermediate between 

 the normal second and third (Fig. 4.4, second row). 



This same experiment also shows that the orientation of the spindle is 

 not the only factor concerned in the cleavage pattern. In the Paracentrotus 

 egg the vegetative region after a certain time has a tendency to form very 

 small cells (micromeres), and will do so whatever the orientation of the 

 spindle which initiates the division (Fig. 4.4, row 3). Again, spontaneous 

 constrictions, mimicking the early stages of cleavage, are seen in the 



Figure 4.4 



Delay of cleavage relative to the orientation of the spindles. Upper row, the 

 normal cleavage of the echinoderm Paracentrotus. Middle row, cleavage 

 somewhat delayed. Lower row, cleavage further delayed, the first cleavage 

 not occurring till the spindle is orientated vertically. (From Horstadius 



1937.) 



