142 



FERTILIZATION OF THE OVUM 



tosoon, on the other hand, is provided with a centrosome, but lacks the 

 substance in which tJiis organ of division may exert its activity. 

 Through the union of the two cells in fertilization all of tJie essential 

 organs necessary for division are brought together ; the egg now contains 

 a centrosome wldcJi by its own division leads the way in the embryonic 

 development}- Boveri did not actually follow the disappearance of 

 the egg-centrosome, but nearly at the same time this process was 

 carefully described by Vejdovsky in the case of a fresh-water annelid 

 Rhynchelmis. Here, again, very strong evidence was brought for- 



Pig. 72. Fertilization of the egg in the copepod Cyclops strenuus. [RUCKERT.] 

 A. Sperm-nucleus soon after entrance, the sperm-aster dividing. B. The germ-nuclei ap- 

 proaching; cf , the enlarged sperm-nucleus with a large aster at each pole; 9, the egg-nucleus 

 reformed after formation of the second polar body, shown at the right. C. The apposed reticular 

 germ-nuclei, now of equal size; the spindle is immediately afterwards developed between the two 

 enormous sperm-asters ; polar body at the left. 



ward to show that the cleavage-amphiaster arises by the division of 

 a single sperm-aster. Very numerous observations to the same effect 

 have been made by later observers. Bohm could find in Petrotnyzon 

 ('88) and the trout ('91) no radiations near the egg-nucleus after the 

 formation of the polar-bodies, while a beautiful sperm-aster is devel- 

 oped near the sperm-nucleus and divides to form the amphiaster. 

 Platner ('86) had already made similar observations in the snail 

 Arion, and the same result was soon afterwards reached by Brauer 

 ('92) in the case of Rranchipus, and by Julin ('93) in Styleopsis. 

 Pick's careful study of the fertilization of the axolotl ('93) proved in 



1 '87, 2, p. 155- 



