CELL-DIVISION AND DEVELOPMENT. 



137 



toplasm by means of the protoplasmic network. The egg at 

 this stage is to be regarded as a multinucleate cell or, as such 

 a structure is usually termed, a syncytium. Now it will readily 

 be seen that in such a case as this the spindles, lying as they 

 do in the center of the undividing mass of the egg, are pro- 

 tected to a very great extent from external influences and 

 especially from mutual pressure, since the mass of protoplasm 

 surrounding each one is in connection with that surrounding 

 any of the others only by the filaments of the network. 



What, then, as to the directions which the karyokinetic 

 spindles assume ? Do they arrange themselves as they should 

 for perfectly rectangular divisions or do they present in some 

 cases special directions ? These questions may be answered 

 by a consideration of the directions assumed by the spindles 

 occurring during the first three cleavages, and for convenience 

 the cleavage will be spoken of as affecting only the nucleus, 

 though it must be remembered that the mass of protoplasm 

 surrounding each nucleus is also affected by it. 



The first cleavage produces two nuclei, the spindles lying at 

 right angles to that one which produces the second polar 

 globule. The second cleavage produces four nuclei, the 

 spindles being directed at right angles to that of the first 

 cleavage, and up to this point the process is in harmony with 

 the third factor of Sachs' law. 

 At the close of the second 

 cleavage a peculiar rearrange- 

 ment of nuclei occurs, two of 

 them, which come from the 

 same spherule of the preceding 

 stage, rotating through an arc 

 of 90 so that the line joining 

 them lies in a plane at right 

 angles to that in which the line 

 joining the other pair lies (Fig. 

 6). I speak of a rotation through 

 90 of one of the pairs of nuclei ; 

 this is merely for convenience, since it is quite possible that 

 both pairs may rotate in opposite directions through arcs of 45, 



FIG. 6. 



