194 On Fertilisation, and the Segmentation of the Spore in Fucus. 



oogonial mitoses, is intranuclear, and it is often separated from, the 

 well-defined persistent nuclear wall by a clear space. The chromo- 

 somes, when assembled on the spindle, at the equator, are seen to be 

 twice as numerous as in the oogonial nuclei, i.e.., seen in profile we 

 counted them as twenty in number. We were unable to distinguish 

 any such grouping of the chromosomes as would lead to the conclu- 

 sion that the chromosomes of the mate and female nuclei respectively 

 had so far preserved their original identity as to appear in the form of 

 two separate groups. The long interval of time which, in Fucus, 

 elapses between fertilisation and the first nuclear division possibly 

 may admit of a more thorough mingling or fusion of the parental 

 chromosomes than would seem to be the case in some animals, e.g., 

 the Copepoda as described by Riickert and by Hacker. 



During the diaster stage the connecting achromatic fibres are at first 

 very distinct, but they soon become fainter, and no cell-plate is 

 formed across them. The two daughter nuclei gradually pass into 

 the state of rest, each being first hemispherical, with crenate projec- 

 tions on the flattened side turned towards its sister nucleus. Only 

 after nuclear division is complete does the first cell wall appear. The 

 cell is sometimes spherical when this happens, and then it is divided 

 into two similar hemispheres. Further divisions may then appear, 

 whilst the general contour of the embryo still remains more or less 

 spherical. These cases occurred most frequently when the germinat- 

 ing spores were illuminated on all sides. But most commonly the 

 first cell wall cuts the spore into two dissimilar halves, one of which 

 grows out and forms a rhizoid. Often this projection is already 

 apparent even before the first nuclear division occurs, and in any 

 case one of the two daughter nuclei always passes down into the 

 protuberance. 



The immediately succeeding divisions have been sufficiently de- 

 scribed by Thuret and others, but we may remark that the division 

 of the nuclei in all cases precedes the formation of a cell plate, which 

 is not formed in connexion with the achromatic connecting fibrils as 

 in the higher plants. 



The doubled number of the chromosomes is retained during the 

 vegetative divisions of the thallus, and is constant throughout the 

 somatic cells of the mature Fucus plant. Hence it follows that the 

 reduction in the number of the chromosomes (in the female plants), 

 is associated with the differentiation of the oogonium the mother cell 

 of the sexual products. Thus Fucus, in this respect, approximates 

 more closely to the type of animal oogenesis than to that which obtains 

 in those higher plants in which the details of chromosome reduction 

 has been followed out. 



Regarded from the standpoint of the number of its chromosomes, 

 the Fucus plant resembles the sporophyte of the higher plants, whilst 



