36 



number of chromosomes, Boveri's results, so far as the division of 

 the enucleated fragments of eggs when fertilized is concerned, have 

 been confirmed by several investigators, and there would seem to be 

 no doubt of the correctness of the observation. 



The facts thus established that in some cases among animals the male 

 and female elements remain separate and almost entirely distinct 

 through several divisions, and that the male pronucleus ma}^ in some 

 cases form a spindle and divide alone without fusing with the female 

 pronucleus, would seem to strengthen the probability that the second 

 male nucleus which enters the embr3''o sac in maize may divide without 

 uniting with the embryo sac nucleus, and thus give rise to the varie- 

 gated or mosaic endosperm, as suggested by the writer. Again, the 

 very recent observations of Guignard ^ on double fecundation are sug- 

 gestive in connection with this hypothesis, as in Endymion he finds 

 that the two polar nuclei, the union of which produces the nucleus of 

 the embryo sac, approach and touch each other long before the pene- 

 tration of the pollen tube into the ovule. But, although flattened at 

 the point of contact, they do not fuse, their contours remaining quite 

 distinct. It would seem quite probable that in some plants these two 

 nuclei may fuse, or even divide, before the male nuclei enter the 

 embryo sac, or be so far advanced that the male nucleus would not 

 fuse with them, and thus result in its remaining isolated and dividing 

 without fusing with a nucleus of female origin. It is of course cer- 

 tain that the fecundation of the embryo sac nucleus is not necessary 

 to the formation of the endosperm in all cases, as in some plants the 

 endosperm is known to form before fecundation takes place. So far 

 as the writer is informed it is not known when the polar nuclei in corn 

 fuse, but it is interesting to note that in Triticum, a related plant, 

 Koernicke found that the polar nuclei approach each other and appar- 

 ently become thoroughly fused together before fecundation takes 

 place. Koernicke wrote: 



Wir erinnern uns, dass vor der Befruchtung die beiden Polkerne des Embryo- 

 sacks sich zum sekundiiren Embryosackkern vereinigt haben. Dieser Doppelkern 

 l)eginnt nach einiger Zeit, wobei man noch immer einen zarten Streifeu bemerken 

 kaiin, der auf seine Entstehung durch Yerschmelzmig eweier Kerne hindeutet, sich 

 zu theilen. Die erste Theilung tritt gewobnlich schon dann ebi, wenn der PoUen- 

 schlauch im Embryosack angekommen ist und sich an die Eizelle angelegt hat. 

 Nachdem die Befruchtmig durch die Vereinigung des Spermakerns mit dem Eikerne 

 vollzogen ist, hat der Endospermkern sich schon mehrfach getheilt.^ 



^ Guignard. In report of the March 12, 1900, meeting of the Paris Academy of 

 Sciences. Nature, Vol. 61, p. 507. 22 March, 1900. 



^ Koernicke, Max. Untersuchung iiber die Entstehung und Entwicklung der Sex- 

 ualorgane von Triticum. Verhand. d. naturhist. Ver. d. preuss. Rheinlande Westf . u. 

 d. Regierungsbezirks Osnabriick, Bd. 53, Jahrgangl896, p. 174. "We must remem- 

 ber that before fecundation the two polar nuclei of the embryo sac have united to 

 form the secondary embryo sac nucleus. This double nucleus begins to divide after 



