THE MORPHOLOGY OF FERTILIZATION 81 



and many others, in appropriate concentrations for 

 proper periods of time. Moreover, a very heavy insemi- 

 nation of any normal lot of eggs will usually yield a small 

 percentage of polyspermy. The number of spermatozoa 

 that may enter under such circumstances may vary from 

 two to a considerable number. The first student of this 

 subject, Fol, in 1877, determined for the starfish and 

 sea urchin that polyspermic eggs divide in more than 

 two cells at the first cleavage and their subsequent 

 development is never normal. 



In 1887 O. and R. Hertwig published a detailed 

 study of polyspermy in the sea urchin; each sperm 

 nucleus forms an aster which subsequently divides to 

 form an amphiaster. If only two sperm nuclei are 

 present both unite with the egg nucleus, and the two 

 amphiasters produce a four-poled karyokinetic figure, 

 or tetraster; the egg divides simultaneously into four 

 cells, but the subsequent division of the cells is always 

 in two each. Triasters sometimes form, owing to fusion 

 of two asters, and a simultaneous division of the egg 

 into three cells follows. If more spermatozoa enter, all 

 sperm nuclei do not necessarily unite with the egg 

 nucleus; two or more may unite with the egg nucleus 

 and a multipolar figure results ; the other sperm amphi- 

 asters then associate themselves with this figure and 

 very complex karyokinetic systems result. 



In the frog (Herlant, 1911), each sperm nucleus forms 

 an aster, as in the sea urchin, but the egg nucleus unites 

 with only one of the sperm nuclei; the supernumerary 

 sperm nuclei form karyokinetic figures also. Thus 

 in the case of dispermy two karyokinetic figures result, 

 one of which contains the chromosomes of the egg and 



