Cytological Problems. 233 
This is to be explained by the progressive division of labour among 
the hereditary units in relation to the increasing complexity of the 
soma. The protoplasm which first appeared on the earth must 
have been without a nucleus, and with the formative and nutritive 
functions equally distributed in all its parts. The portions of 
protoplasm set apart for formative functions were the first hereditary 
units and they remained at first distributed in the general cytoplasm, 
as we still find them in certain lower organisms which have no 
definite nucleus, but only a number of discrete chromatin particles. 
By the next step they became united in a definite association without 
being sharply separated off from the general protoplasm, a condition 
perhaps found in certain bacteria and Cyanophycete. In the next 
stage the units became sharply marked off from the general 
protoplasm and we have a definite distinction of nucleus and 
cytoplasm. At this stage the hereditary units, though in part 
different among themselves, would also be in part the same, i.e., the 
same purpose would be served by several units. In this condition 
a division of the nucleus in a relatively simple manner, such as 
fission, would be sufficient to give a full complement of hereditary 
units to each daughter nucleus. As the difference, however, between 
the units increased, till finally the stage was reached in which each 
hereditary unit had a special function, this mode of division would 
be clearly insufficient. The qualitative equal division of the nucleus 
had then to be brought about by the arrangement of the hereditary 
units (pangens) in thread-like structures; in these threads the units 
divide and then by the splitting of the thread, a separation of the 
products of division would be brought about. This mode of division 
would seem to be the only possible one for the purpose, for otherwise 
the higher animals and plants would not show so complete an 
agreement in the process. The necessity for the longitudinal 
arrangement of the pangens led to the production of chromosomes, 
and in passing up the vegetable kingdom we appear to find a 
transition from a large to a small number of these structures; this 
is well seen in passing from vascular cryptogams to phanerogams. 
Ferns show very high numbers, while the highest groups of 
Angiosperms often show exceedingly low numbers. The combination 
of numerous short threads into fewer longer threads would probably 
have certain advantages in the formation of the mitotic figure. 
The wide range of cytological matters dealt with in this paper 
and the stimulating nature of the observations and views put forward 
show clearly that the veteran cytologist still retains unimpaired 
that mental elasticity—the power to accept and produce new views 
—which is the mark of intellectual youth. 
V.H.B. 
