THE MORPHOLOGY OF FERTILIZATION 73 



In those cases in which the history of the sperm 

 aster is clearly shown throughout (echinids, many 

 annelids and mollusks, ascidians, etc.), the central 

 body of the aster soon divides in two centrosomes which 

 move apart with consequent formation of an amphi- 

 aster (Figs. 4, 7, 8), which becomes the achromatic 

 part of the first cleavage spindle. In echinids (Fig. 7), 

 in which maturation is completed and the egg nucleus 

 formed prior to insemination, the division takes place 

 about the time of meeting of the germ nuclei, for there 

 are no preparatory changes remaining to be accom- 

 plished by the egg and the two nuclei therefore unite 

 very rapidly. The plane of the separation in this case 

 is at right angles to the line uniting the centers of the 

 two germ nuclei. In those cases in which the egg 

 has part of the maturation process still to complete 

 the sperm amphiaster remains more or less quiescent 

 during maturation (Fig. 8), but it may entirely dis- 

 appear (Unio) or diminish to a variable extent (e.g.. 

 Nereis), and the asters of the first cleavage spindle 

 are to this extent new formations, though certainly in 

 some cases, and possibly in others, formed around the 

 original centers. 



The morphological variations are very numerous 

 with respect to the genetic behavior of the first cleavage 

 amphiaster when we compare all the various species 

 studied. As the physiological foundation must be sup- 

 posed to be uniform we can interpret the morphological 

 variations as due to time differences in the components 

 of the physiological reactions in different species. 



When we find that the sperm nucleus behaves dif- 

 ferently in respect to aster formation from the egg 



