1 62 FERTILIZATION OF THE OVUM 



of the pollen-tube, and its growth down through the tissue of the 

 pistil to the ovule, was observed by Amici ('23), Brogniard ('26), 

 and Robert Brown ('31); and in 1833-34 Corda was able to follow 

 its tip through the micropyle into the ovule. 1 



Strasburger ('77-88) first demonstrated the fact that the generative 

 nucleus carried at the tip of the pollen-tube enters the ovum and 

 unites with the egg-nucleus. On the basis of these observations he 

 reached, in 1884, the same conclusion as Hertwig, that the essential 

 phenomena of fertilization is a union of two germ-nuclei, and that 

 the nucleus is the vehicle of hereditary transmission. Strasburger 

 did not, however, observe the centrosome in fertilization. This was 

 accomplished in 1891 by Guignard, who demonstrated in the case of 

 the lily (Lilium Martagoii) that the generative nucleus as it enters 

 the egg is accompanied by a small quantity of cytoplasm and by two 

 centrosomes (Fig. 80). He showed further that the egg also con- 

 tains two centrosomes ; and according to his account the conjugation 

 of the nuclei is accompanied by a conjugation of the centrosomes, as 

 already described. 



Guignard also first cleared up the history of the chromosomes, 

 reaching results closely in accord with those of Van Beneden in the 

 case of Ascaris. The two germ-nuclei do not actually fuse, but 

 remain in contact, side by side, and give rise each to one-half the 

 chromosomes of the equatorial plate, precisely as in animals (Fig. 80). 

 The number of chromosomes from each germ-nucleus is, in the lily, 

 twelve. The later history is identical with that of the animal egg, 

 each chromosome splitting lengthwise, and the halves passing to 

 opposite poles of the spindle. Each daughter-nucleus therefore 

 receives an equal number of chromosomes from the maternal and 

 paternal germ-nuclei. 2 



As in the case of animals (p. 127), the germ-nuclei of plants show 

 marked differences in structure and staining-reaction before their 

 union, though they ultimately become exactly equivalent. Thus, 

 according to Rosen ('92, p. 443), on treatment by fuchsin-methyl-blue 



1 It is interesting to note that the botanists of the eighteenth century engaged in the same 

 fantastic controversy regarding the origin of the embryo as that of the zoologists of the 

 time. Moreland (1703), followed by Etienne Francois Geoffroy, Needham, and others, 

 plaged himself on the side of Leeuwenhoek and the spermatists, maintaining that the pollen 

 supplied the embryo which entered the ovule through the micropyle. (The latter had been 

 described by Grew in 1672.) It is an interesting fact that even Schleiden adopted a similar 

 view. On the other hand, Adanson (1763) and others maintained that the ovule contained 

 the germ which was excited to development by an aura or vapour emanating from the pollen 

 and entering through the tracheae of the pistil. 



2 Guignard's observations on the conjugation of the centrosomes have already been con- 

 sidered at p. 159. They stand at present isolated as the only precise account of the history 

 of the centrosomes in plant-fertilization, and no general conclusions on this subject can 

 therefore at present be drawn. 



