in 



SYNGAMY 77 



round the egg almost instantaneously after the entrance of the first 

 spermatozoon. 



In some eggs, however, more than one spermatozoon always enter at 

 fertilization {polyspermy) , though in no case does more than one normally 

 fuse with the egg nucleus. In some cases the supernumerary spermatozoa 

 merely degenerate and are absorbed (Axolotl, Fick, 1893) ; in other cases 

 they maintain themselves for a considerable time in the egg cytoplasm, 

 even forming nuclei which may increase in number by amitotic fragmenta- 

 tion {Triton, Braus, 1895). In yet other types the supernumerary sperma- 

 tozoa metamorphose themselves into nuclei which multipty by mitosis 

 (exhibiting of course the haploid number of chromosomes) and may persist 

 for a long time in development. In the case of the Elasmobranchs, where 

 this type of polyspermy has been most thoroughly studied (Riickert, 

 1899), the ultimate fate of these nuclei is not known, but it is extremely 

 improbable that they take any part in the formation of the embryo. 

 In the pigeon, the nuclei derived from the extra spermatozoa, which 

 also multiply by mitosis, disappear much earlier, namel^^ about the 32-cell 

 stage (Blount, 1909). 



Cases such as the above, where several spermatozoa normally enter 

 the egg (though only one fuses with the egg nucleus), are said to exhibit 

 physiological polyspermy. This is specially characteristic of large, heavily 

 yolked eggs (Insects, Elasmobranchs, Amphibia, Reptiles, Birds). In 

 other cases, however, the entry of more than one spermatozoon into the 

 egg is pathological {pathological polyspermy) and leads to abnormal 

 development, owing to the multiphcation of centrosomes and to the fact 

 that more than one of the microgamete nuclei enters into relation with 

 the female nucleus. If two spermatozoa enter the egg of Ascaris megalo- 

 cephala or of Echinus, both male nuclei form chromosomes and both give 

 rise to a pair of centrosomes. Thus there are three sets of chromosomes 

 (two 6 and one ? ) and four centrosomes. A four-poled spindle figure 

 is thus produced, and at the first cleavage division the egg divides simul- 

 taneously into four blastomeres instead of two, the 3^ chromosomes being 

 irregularly distributed among the four nuclei. These special cases are 

 more fully described later on (p. 162). 



C. GONOMERY 



We have seen that the gamete nuclei may fuse in the resting condition, 

 or may carry through the prophases of mitosis while still separate, in 

 which case the chromosomes of the two gametes come together for the 

 first time on the first cleavage spindle. Sometimes, however, the two 

 nuclei retain their individuality much longer (Fig. 35). In various species 

 of Cyclops, for instance (Riickert, 1895 ; Hacker, 1895), the resting nuclei 



