vm] POLYSPERMY 123 



the egg-surface impermeable to other spermatozoa even if 

 the membrane be broken. If, however, the vitality of the 

 egg is impaired by treatment before fertilisation in various 

 ways (e.g. with certain alkaloids, heat, or long standing in 

 water) the formation of the membrane is delayed and more 

 than one spermatozoon may enter, and in consequence a 

 more or less abnormal fertilisation process takes place. The 

 simplest and commonest case is that when two spermatozoa 

 are present in the egg. When this happens both spermato- 

 zoa give rise to nuclei and centrosomes, and usually each 

 centrosome divides, with the result that at the first seg- 

 mentation division there are four centrosomes and three 

 times the .reduced number of chromosomes. Not rarely 

 one of the centrosomes fails to divide, so that there are 

 three instead of four. In the first case the division figure 

 takes the form of a tetraster, a figure having four poles with 

 a spindle arranged between each and the next, forming 

 roughly the sides of a square; some observers maintain that 

 there are also diagonal fibres running across the middle of 

 the square from corner to corner; others deny this. When 

 there are only three centrosomes the figure is a triaster con- 

 sisting of three spindles arranged in a triangle (cf . Text-fig. 6, 

 p. 57). The chromosomes become arranged on the spindles 

 rather irregularly, and some may be omitted altogether and 

 remain lying in the centre. Those which take their places 

 on the spindles divide normally, and their halves pass to 

 the poles in the usual way, thus giving rise to three or four 

 nuclei near the centrosomes. The egg then divides simul- 

 taneously into as many blastomeres as there are nuclei, 

 and development may proceed apparently normally for a 

 considerable period, though in the later embryo and larva 

 abnormalities always appear. 



A number of interesting problems centre round these 



