236 
Allen. — Nuclear Division in the 
These attempts to explain chromosome reduction by a separation of 
entire chromosomes are based, as has been said, upon what appear to be 
the facts particularly among the Arthropods. For those cases in which 
a double longitudinal splitting is observed, and in which, therefore, both 
heterotypic and homoeotypic divisions seem to be equational, it has been 
found much more difficult to frame a satisfactory hypothesis. Such a double 
longitudinal splitting was described by Brauer (’93) in the spermatogenesis 
of Ascaris ; and a similar process has been found by various authors in 
widely divergent groups of plants and animals, particularly among the 
Vertebrates (Flemming, ’87 ; Moore, ’95 b ; Meves, ’96 ; McGregor. ’99 ; 
Kingsbury, ’99, &c.) ; and among the Seed Plants (Strasburger, ’95, *00 ; 
Mottier, ’03 ; Guignard, ’99 ; Gregoire, ’99, and others). 
O. Hertwig (’90) conceived of the fusion of the sexual cells as involving 
a fusion of the parental idioplasms in such a way as to give rise to a new 
and different idioplasm characteristic of the new individual. The divisions 
resulting in the formation of the germ-cells, he thought, are equation 
divisions, and the germ-cells formed by the same individual are all similar. 
Strasburger (’94) thought that in the prophases of the heterotypic division 
each ‘ id ’ derived from one parent fuses with an ‘ id ’ derived from the other 
parent, the result being a new idioplasm which is equationally divided 
among the germ-cells. Brauer seems to have considered that no such 
fusion is necessary, but that the two parental idioplasms remain and are 
simply reduced in mass by two equation divisions. Montgomery (’01), 
Sutton (’03), and Boveri (’04) have suggested that the phenomena in 
Vertebrates may be harmonized with those in Arthropods by supposing 
that in the former the chromosomes conjugate side by side instead of end 
to end, and that one of the apparent longitudinal splittings is, therefore, 
really a separation of two independent somatic chromosomes. This 
suggestion is an approximation to the facts as I have described them in the 
lily ; but the fusion of the two spirems into a single thread, as I have 
observed it, is a very different process from a temporary attachment of the 
chromosomes end to end ; and the two methods may be expected, if they 
actually occur in different groups of organisms, to lead to very different 
results as regards the transmission of hereditary characters. 
It seems quite possible from the figures of Brauer (’93) that the 
occurrences in Ascaris are really very similar to those in the lily. He 
finds that at about the beginning of the synaptic period certain strands 
of the nuclear network appear double, while at the close of synapsis there 
is present a continuous doubly-split spirem. He thinks it probable that 
the two longitudinal fissions are nearly or quite simultaneous, and that, 
owing to their fineness in the earlier stages, the strands appear double 
when they may really be four-parted. The double longitudinal splitting 
of the spirem certainly occurs earlier in Ascaris than in the lily; but 
