148 CYTOLOGY 



CHAP 



Multiplication of chromosomes has been obtained by experimental 

 methods by N^mec (1904, 1910). If the root tips of plants {Vicia, 

 Pisum, Allium) are subjected to the narcotizing action of chloral hydrate, 

 any ceU divisions that are in progress at the time are inhibited to this 

 extent, that the telophase reconstruction of the nuclei takes place, but 

 cell division is not completed. Thus binucleate cells are formed. If 

 the root tips are allowed to recover from their narcotization, the 

 two nuclei, which He close together, may fuse into a single giant 

 nucleus. This may proceed to mitosis, in which case it exhibits 4W 

 chromosomes. 



If the root tips are repeatedly narcotized, being allowed to recover 

 after each narcotization, the above process may also be repeated, resulting 

 in giant nuclei with 8« or probably even i6w chromosomes. 



From the fact that such polyploid nuclei become fewer in proportion 

 to the length of time that has elapsed since the plants recovered from 

 the narcotization, Nemec concludes that a reduction to the normal 

 number of chromosomes ultimately takes place, and indeed found certain 

 mitoses which he beUeved to be the reduction divisions ; this interpreta- 

 tion is, however, necessarily uncertain. 



There are many cases known where of two nearly allied species one 

 has double the number of chromosomes of the other, and the possibiUty 

 naturally suggests itself that the chromosome number may have been 

 doubled in this way, namely, by a fission of the chromosomes unaccom- 

 panied by nuclear division. Forms thus derived would obviously have 

 four sets of chromosomes ; that is to say, they would be teiraploid 

 instead of diploid in their somata, and diploid instead of haploid in their 

 gametes. It must not, however, be assumed without special evidence 

 that any species is tetraploid, since in the great majority of cases where 

 one species has twice as many chromosomes as nearly related species, 

 the larger number has almost certainly been derived from the smaller 

 either by the aggregation of the chromomeres into double the number 

 of chromosomes or by the transverse fragmentation of all the chromo- 

 somes into two — as, for instance, happens in the somatic cells of Ascaris 

 canis and the other examples mentioned on p. 143. If transverse con- 

 strictions in chromosomes are signs of a weak point where fragmentation 

 is Uable to take place, there is no difficulty in understanding how the 

 nmnber of chromosomes comes to be exactly doubled, for it may happen 

 that every chromosome exhibits this constriction {e.g. Lepidosiren, p. 40), 

 and there are very few references in Uterature to a chromosome exhibiting 

 more than one such transverse joint. 



Out of the numerous cases of numerical doubhng of chromosomes 

 known, the following few striking examples may be quoted : 



