368 METAMORPHOSIS 



division. In the normal vegetative division the chromosomes, which occur in 

 a definite and, for each species, perfectly constant number, split longitudinally, 

 so that each daughter-cell contains the same number as the parent-cell. In the 

 division which precedes the formation of the sexual-cells a similar number of 

 chromosomes appears, but since they do not split longitudinally, both ovum and 

 sperm contain only half the usual number and only by fertilization is the full 

 complement again attained [compare Fig. 87 in the 7th ed. of the Bonn Text- 

 book]. A phenomenon similar to this reduction is also found in Phanerogams. 

 The nucleus in the embryo-sac and that in the pollen-grain show only half as 

 many chromosomes as do the vegetative cells. The reduced number is not 

 brought about, however, by suppression of the longitudinal division but by 

 the nuclein thread dividing into only half the regular number of chromosomes. 

 [Since these words were written a very comprehensive summary of the cyto- 

 logical literature has been published, showing that a reduction-division occurs 

 before the formation of sexual-cells both in plants and in animals (compare 

 KoRNiCKE, 1904 ; Strasburger, 1904 a, 1905, &c.] 



Among zoologists the arrest of the developmental process has been 

 attributed to this reduction, and many botanists, e.g. Juel (1900 b), hold the 

 same view as to the reduced number of chromosomes. Apart altogether from the 

 fact that the reduction in the number of chromosomes is brought about in quite 

 different ways there are other considerations which militate against this view. 

 In speaking of the genetic relationships between Pteridophyta and Phane- 

 rogams in the foregoing lecture we saw that the whole of one generation of the 

 fern — namely, the prothallial generation, on which the sexual organs are borne — 

 is reduced to a few cell divisions in the spores of the Phanerogams. In ferns 

 the prothallial generation possesses the half number of chromosomes and yet 

 it is capable of vigorous development. Chromosome reduction in the Phane- 

 rogams is manifestly an ancestral character handed down from earlier types — 

 perhaps fern-like in character — and has no direct relation to fertilization, as 

 appears to be the case in animals. Also in the case of apogamy in ferns, in 

 which the sporophyte arises on the prothallium without the intervention of an 

 ovum it seems that the increase in number of the chromosomes can come about 

 otherwise than by fertilization, for it can hardly be doubted that apogamous 

 shoots have the same number of chromosomes as the normal one. In apogamous 

 ferns, according to Farmer, Moore, and Digby (Proc. Royal Soc, 1903, 71, 

 453), a fusion between the nuclei of two vegetative cells takes place, leading to 

 a doubling of the chromosomes, and the nucleus resulting forms the starting-point 

 for the apogamous growth. We ought, perhaps, to receive this statement with 

 caution, and may raise the question whether the authors determined that such 

 a fusion takes place only in apogamous prothalli, and also that such a fusion 

 actually does serve as the starting-point for the new growth. Can the passage 

 of a nucleus from the one cell to the other have been the result of the mode of 

 preparation as in Miehe's observations quoted on p. 170 ? [Farmer has as 

 yet given no further explanation, but Digby (1905) has published some 

 additional preliminary notes to which reference may be made. Nemec (1904) 

 also appears to incline towards the interpretation we have given above of the 

 significance of the transference of the nucleus.] 



For other consequences of the chromosome hypothesis we must refer to 

 p. 377, and turn now to the consideration of another theory of fertilization 

 which finds its chief advocate in Boveri (1902). This hypothesis has grown out 

 of investigations carried out upon animals. The egg is supposed to be in- 

 capable of growth because it possesses no ' centrosome ', and the influence of 

 the sperm depends not on its bringing a nucleus but a centrosome to the ovum. 

 Hitherto we have not spoken of this organ of the cell, simply because it is entirely 

 wanting in the highest plants. The animal cell, on the other hand, very 

 generally contains small granules surrounded by protoplasm exhibiting radiating 



