THE NUCLEUS IN MATURATION 



17 



by the brilliant experiments of Boveri, Delage, and others, that portions of sea-urchin eggs broken 

 by shaking, or cut into fragments (merogoriy), which contain no part of the germ-nucleus, 

 may, when fertilised by spermatozoa, divide, and ultimately form larvae. The sperm 

 nucleus is thus sufficient by itself for the segmentation of the egg, a centre- 

 some being introduced or produced in the protoplasm. Again, it has been shown, 

 first by Loeb and then by many others, that the eggs of echinoderms and other invertebrates 

 may be made to segment and form Iarva3 by treatment, in various ways, with certain chemical 

 substances, by shaking and so forth, without the influence of the spermatozoon. The egg- 

 nucleus is thus sufficient in itself for segmentation and development of the 

 egg when, by artificial means, a centrosome is produced in the protoplasm. 1 



Thus we reach the general conclusion that the union of the nuclei is not the means by which 

 the developmental process is started, but nevertheless it is the essential factor in fertilisation 

 in short, it is the end and aim of the process. The union of the paternal and 

 maternal chromatin (amphimixis] is the all- important fact, and for this reason, that (without 

 denying to the protoplasm a certain influence) the chromatin of the nucleus is the material 

 basis of the hereditary qualities handed on from one generation to another. 



Reduction of chromatin. It is sufficiently obvious that if there is a 

 fusion of paternal and maternal chromatin in fertilisation at each generation 

 the amount of chromatin would be doubled, 

 on the assumption that the mass is con- 

 stant in all the nuclei of each generation. 

 The necessary reduction is effected during 

 the maturation of the sexual cells. 



Before entering on a description of the process of 

 reduction, it is necessary to refer briefly to two related 

 hypotheses as to the constitution of the nucleus. 



It is now practically certain that the number of 

 chromosomes, is constant in each species, and that out 

 of a resting nucleus the same number of chromosomes 

 emerge as entered it at the end of the preceding 

 division. It is believed by some that the chromo- 

 somes retain their identity in the resting nucleus, so 

 that the chromosomes which emerge from it are the 

 same as those which entered it.- As a corollary to 

 this theory of the persistent identity of the chromo- 

 somes, there is a second hypothesis that the paternal 

 and maternal chromosomes, equally distributed 

 between the two first blastomeres (fig. 27), have a 



separate and persistent identity in all the cells of the soma, and consequently in the spermato- 

 gonia and oogonia. 



Nuclear phenomena during 1 development of the sexual cells. The 



history of the sexual cells in respect of the nuclear changes may be divided into 

 three phases a pre-reduction, a reduction, and a post-reduction phase. 3 The pre- 

 reduction phase includes all the cell-generations up to that of the spermatogonia and 

 oogonia (fig. 22). During this period the nuclei behave in all respects like the nuclei 

 of somatic cells, arid possess the same number of chromosomes. The reduction 

 phase involves the generations known as spermatocytes and oocytes of the first and 

 second orders, and is characterised by two divisions differing in their characters from 

 all other varieties of mitosis, during which the number of chromosomes is 

 reduced to one-half of the somati c number. The first division is known as 



1 Further information as to the observations here referred to will be found in a review of the literature 

 in the Quarterly Journal of Microscopical Science, xlvi. 1902, by T. H. Bryce. 



2 It appears to me that the facts of maturation of the ovum, as will be seen later, form an obstacle to 

 the present unrestricted acceptance of the theory of the persistent identity of the chromosomes in its 

 crude form, but that, notwithstanding, it is necessary to assume a segregation in the chromatin mass, 

 which may, as it were, crystallise at each division into chromosomes of specific characters. T. H. B. 



5 For these phases the terms pre-meiotic, meiotic, and post-meiotic are employed by Moore and 

 Walker (derived from peiou, to make smaller). 



VOL. I. O 



FIG. 26. FERTILISATION ; EGG OF BAT. 

 (Van der Stricht.) 



S.n., sperm-nucleus ; g.n., germ-nucleus ; 

 P.6., polar bodies. 



