136 



GENERAL PRINXIPLES OF ZOOLOGY 



Part Played by the Two Nuclei. — In many cases an alilirevialion of 

 development may take place, the stage of the cleawige nucleus being omit- 

 ted, and the egg and sperm nuclei, without uniting, pass directly into the 

 cleavage spindle. Tliis in no wise alters the alxne-mentioncd proposition, 

 but yet it is important, because it shows more plainly how the two nuclei 



D 



Fig. 07. — Four stages in the fertilization of Slroii(;ylocfnlrolus lividns (after 

 Kostanecki"). .4, entrance of sperniatozoon; B, turning of sperm nucleus; C, ap]iroach, 

 and D, fusion of egg and sperm nuclei. (In .1 and B only a piart of the egg is shown.) 



participate in the formation of the cleavage spindle. It shows that of 

 the chromosomes wliich form the equatorial plate exactly one-half are fur- 

 nished b)'the egg-nucleus, the other by the sperm-nucleus. For, c\'en lie- 

 fore the spindle has been formed and the contour of the two nuclei has 

 disappeared, the chromosomes destined for the spindle are completely 

 developed in exactly the same number in each of these (tig. gS). 



Ftp,. q8. — Fertilization of Asraris vir^aloccphalci (after ISoveri"). .-1, the ends 

 (ccntrosomes) of the spindle fni-nied; B, the spindle comi)lcted; ,?/), sperm-nucleus with 

 its chromosomes; c/, egg-nucleus; p^ polar bodies. 



Heredity. — Recent observations ha\'e furnished a certain basis for 

 the theory of heredity, the transmission of parental characteristics to the 

 offspring. This Iransnussioii, on the whole, takes place with eijual effect 

 from the father's and from the mother's side; if we take the a\'erati:e of 



