20 BIOLOGICAL LECTURES. 



bodies, the spJieres directrices, which are the equivalents of 

 Van Beneden's spheres attractivcs and Fol's asters ; two of 

 these spheres are also found in contact with the female pro- 

 nucleus, Fig. i K. The two spheres which precede the male 

 pronucleus join those which surmount the female pronucleus 

 in such a way as to form two couples, each of which is com- 

 posed of one element derived from the male cell, the other 

 from the female cell. When the pronuclei come in contact, 

 the two couples diverge until they come to lie at opposite poles 

 of the nuclei, Fig. i L. Then the elements of each couple 

 fuse together into a single sphere with a single central cor- 

 puscle, Fig. i M, and from these spheres the first cleavage 

 spindle is formed. Guignard concludes, therefore, that fertili- 

 zation is not a purely nuclear phenomenon. " It consists not 

 only in the union of two nuclei of different sexual origin, but 

 also in the fusion of two protoplasmic bodies whose essential 

 elements are the spheres directrices of the male cell and of the 

 female cell. Even if the nuclei are of great importance in the 

 transmission of hereditary properties, the permanent presence 

 of spheres directrices in the sexual and somatic cells, and above 

 all their fusion at the moment of fecundation, oblige us to 

 assign to the protoplasm the primordial role in the accom- 

 plishment of this phenomenon. This fusion appertains to 

 the very essence of fertilization ; it is necessary for the 

 formation and subsequent evolution of the egg." 1 



More than a year ago (July, 1892), I found that the eggs of 

 one of our marine Gasteropods, Crepidnla plana, offered excep- 

 tional advantages for the study of the phenomena of fertiliza- 

 tion. In this egg, and that of other species of the same genus, 

 I had been able to trace the cell-lineage to more than one hun- 

 dred cells, and last summer, with more favorable material and 

 better methods, I was able to follow in surface preparations, as 

 well as in sections, the movements of the male and female pro- 

 nuclei, and what is much more important, the whole history of 

 the asters ' 2 from the time the first polar body is formed and the 



1 Loc. cit., p. 276. 



2 For these permanent organs of the cell, known by various authors as the 

 "spheres attractive*" " spheres directrices" " archoplasmic bodies," " periplasts," 



