i6o SCIENCE PROGRESS. 



changes. It was owing to the pecuHarities of these chro- 

 mosomes in the Salamander that Flemming gave the name 

 of heterotype to this mitosis to distinguish it from the 

 nuclear divisions in the rest of the body. The term 

 Heterotype, or Great Heterotype, has now come into com- 

 mon use to designate the first of the two divisions, because 

 although the second may resemble the first one, as in Elas- 

 mobranch fishes according to Moore, ^ it far more commonly 

 differs widely from it, approximating more or less closely 

 to the form typical for the ordinary somatic nuclear divi- 

 sions of the organism, as is the case in the Lily divisions. 

 There are, of course, various forms under which the hetero- 

 type character of division may appear ; but for the most 

 part the net result is clearly a longitiidinal division of the 

 chromosomes followed by a second longitudinal division in 

 the next mitosis. 



I said "for the most part," because there are some 

 apparently well-authenticated exceptions to the general 

 scheme of division which I have just indicated, and they 

 seem to show that in some cases at any rate the pro- 

 cess of reduction may be accomplished in a very different 

 way. In certain Crustaceans, Riickert' and some other 

 investigators state that in the first of the two divisions with 

 which we are concerned it is true that the number of the 

 chromosomes appears to be reduced to one half, and that 

 these undergo longitudinal fission in the usual way ; but 

 each of these single chromosomes is really double, and re- 

 presents two of the original (pre-reduction) chromosomes 

 which have become linked end to end together. The half 

 number then may be said to occur here owing to the incom- 

 plete transverse separation of the linin thread, every other 

 transverse separation having been omitted. But as the 

 young (double) chromosome rudiments continue to grow 

 their duplicate nature reasserts itself, and a sort of separa- 



1 Moore, " On the Structural Changes in the Reproductive Cells 

 during the Spermatogenesis of Elasmobranchs," Quart. Joiirn. Micr. 

 Sa'., vol. xxxviii., new ser. 



- Ruckert, "Zur Eireifung bei d. Copepoden," A/ia/. Ife/ten, heraus- 

 gegeben v. Mei-kel u. Bonnet, 1894. 



