136 



GENERAL PRINCIPLES OF ZOOLOGY 



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

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

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

 cleavage spindle. This in no wise alters the above-mentioned proposition, 

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



D 



A 



FIG. 07. Four stages in the fertilization of Strongylocentrohts Uvidus (after 



L'n c v i (./(. c f 1 1 ' ( '( n .1 f i t (j {Co ^ ill IC1 



Kostanecki). .4, entrance of spermatozoon; B, turning cf sperm nucleus; C, approach 

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



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

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

 nished by the egg-nucleus, the other by the sperm-nucleus. For, even be- 

 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 (fig. 98). 



A 



B 



FiG. q8. Fertilization of A scar is megnloccphala (after Boveri). A, the ends 

 (centrosomes) of the spindle formed; B, the spindle completed; sp, sperm-nucleus with 

 its chromosomes; ei, egg-nucleus; p, polar bodies. 



Heredity. Recent observations have furnished a certain basis for 

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

 offspring. This transmission, on the whole, takes place with equal effect 

 from the father's and from the mother's side; if we take the average of 



