54 



PRINCIPLES OF EMBRYOLOGY 



the egg and the elevation of the fertilisation membrane. After short ex- 

 posures to activating agents these two changes are delayed and there is no 

 sign of any extrusion of polar bodies. Eventually, however, the egg rounds 

 up, the membrane rises and the egg proceeds to cleave, the cleavage 

 spindle being, in fact, that which would normally have given rise to the 

 first polar body. Slightly longer exposure results in what appears to be 

 normal activation. The egg rounds up, the membrane rises and the two 

 polar bodies are formed in normal sequence. The eggs, however, then 

 usually fail to undergo any further cleavage, apparently because they 



Figure 3.4 



Parthenogenesis in the echiuroid worm Urechis. The upper figure shows the 

 unfertilised egg, in which the animal pole is depressed. Columns A, B and C 

 represent the behaviour following increasing exposures to ammoniacal sea- 

 water. Row I, time of first polar body formation; row II, time of second 

 polar body formation; row III, time of first cleavage. (From Tyler 1941.) 



contain only a single aster and not a bipolar spindle. With still longer 

 exposure the rounding up of the egg and the membrane elevation pro- 

 ceed normally, but the first polar body division takes place inside the cell, 

 which is thus provided with two nuclei at each of which a spindle appears 

 ready for the second polar body division. At this second division two, one, 

 or no polar bodies may be extruded, leaving two, three or four nuclei still 

 inside the egg. The eggs then divide into the corresponding number of 

 cells and continue their development to give normal larvae. It is clear in 

 this case that although in some ways the activation is most nearly normal 

 with that treatment which allows the polar bodies to be formed in the 



