1 8 BIOLOGICAL LECTURES. 



Two years ago Hermann Fol ^ published a preliminary account 

 of the fertilization of the egg of a sea-urchin, in which he com- 

 municated the new and remarkable fact that each of the sexual 

 cells, ovum as well as spermatozoon, contains a kinetic center 

 or centrosome, which he called respectively the ovoccntcr and 

 the spcrmoccntcr. When the spermatozoon enters the ovum 

 the spermocenter precedes the nucleus of tlie male cell, 

 Fig. I G, and finally reaches a point on the surface of the 

 female pronucleus opposite the ovocenter ; then the male pro- 

 nucleus comes to lie closely against the surface of the female 

 pronucleus, though it does not intimately fuse with it. The 

 two centers then divide. Fig. i H, and the half-centers push- 

 ing apart move in opposite directions around the cleavage 

 nucleus until one half of the ovocenter meets half of the 

 spermocenter at a point about 90° from that originally occu- 

 pied by the centers, Fig. i I. By the fusion of these half- 

 centres, one derived from the ovocenter the other from the 

 spermocenter, the astroccntcrs are formed. Around each of 

 these astroccntcrs there is formed a clear space, the asirococl, 

 which probably corresponds to the medullary zone of Van 

 Beneden, as the astrocenter corresponds to the central cor- 

 puscle, while the astrocoel is surrounded by a zone of astral 

 radiations, which is probably the cortical zone of \^an Beneden 

 and the archoplasm of Boveri. From these facts regarding 

 the formation of the astrocenters, Fol concluded that " fertiliza- 

 tion consists not only in the adding together of two pronuclei 

 derived from individuals of different sexes, but also in the 

 fusion of four half-centers derived from the father and the 

 mother into two new bodies, the astrocenters." - 



In November of the same year, 1891, (iuignard''^ published 

 a most admirable account of the process of fertilization in some 

 of the flowering plants. He showed that the male pronucleus, 

 which is brought into close proximity to the female pronucleus 

 by the growth of the pollen tube, is always i)receded by two 



1 Hermann l-Ol, Die C"entren(jii;ulrille eine neiie ICpisode aus der ISofruchtungs- 

 geschichte. Aiiat. Anzeiger, May, 1S91. 



2 Loc. cit., p. 274. 



8 Guignard, Nouvelles Etudes siir la I'"t'condation. Aniiaiis i/c's sciences iiatur. 

 Tom. 14, Po/iJiii(/i(c, 1S91. 



