The Early Development of the Pigeon's Egg. 21 



Polyspermy in IlolotJtiu-oldca. — Korschelt and Heider ('03) report 

 a publication by Iwanzoflt' ('98) on polysj)ermy in the Holothuroidea. 

 He found that the egg sends out radiating protoplasmic processes 

 through the canals of the zona radiata. The swarming spermatozoa 

 are caught in these processes and are drawn into the egg plasm, 

 and assimilated by it, as Iwanzoff thinks. "Das Ei frisst und verdaut 

 die Spermatozoen." But Iwanzoff observes that when the egg has 

 lost a part of its nuclear substance, in the polar bodies "die Sper- 

 matozoon konnen nicht mehr von der Eizelle bewaltigt werden," 

 and only one sperm nucleus can unite with the egg nucleus in fertili- 

 zation. Here is one more attempt at a teleological explanation of 

 the maturation process. 



Polyspermy in Insects. — Among the insects, Henking ('92) found 

 polyspermy in certain of the Hemiptera, Coleoptera, and Hymenop- 

 tera. In the egg there is a varying number of micropyles (4 to 7). 

 Each one permits the entrance of a spermatozoon. After fertiliza- 

 tion, the spermatozoa are found in the marginal protoplasm, where 

 they degenerate, bvit they are also found in the yolk where the sperm 

 head changes over into a nucleus. 



Polyspermy in Selacliiajis. — Polyspermy has been described in 

 every class of vertebrates except the mammalia. In the selachians, 

 this phenomenon was discovered in an attempt to determine the 

 origin of the free nuclei in the yolk. For a long time it was supposed 

 that these nuclei were derived from nuclei of the blastomcres of early 

 cleavage, or else that they arose in a still earlier stage by a rapid 

 division of the cleavage nucleus. But further research demonstrated 

 these nuclei in maturation stages before the union of the pronuclei, 

 and thus disproved their descent from the cleavage nucleus. More- 

 over, sperm heads were found in the egg in transitional stages in 

 their process of change into nuclei, and these nuclei in their early 

 mitotic division have the reduced number of chromosomes. 



But while there is now agreement as to the origin of the super- 

 numerary nuclei, their fate is still a question for research. 



Riickert ('99) identifies the merocyte nuclei of late cleavage 

 stages with those of early cleavage which are derived from super- 

 numerary spermatozoa. But he finds the merocyte nuclei in cells 



