226 
Allen. — Nuclear Division in the 
Not only do the chromosomes in all probability differ among them- 
selves in structure and function ; but, in view of the extremely complicated 
facts of heredity, as well as of the pains taken in somatic divisions to ensure 
an exact longitudinal halving of each chromosome, we are forced to think 
of these elements as by no means simple in structure, but rather as having 
a characteristic and complex organization. Observation has shown the 
chromosomes, or at least the spirem-thread before segmentation, to be 
made up as a rule of chromatin bodies (chromomeres) embedded in a less 
chromatic ground-substance (linin). Strasburger (’82) found that the 
chromomeres are formed by a progressive fusion of much smaller granules, 
and this observation has been confirmed in many cases. The chromomeres, 
as I have observed them in the lily, seem quite plainly to be aggregates of 
smaller bodies. Little has been done toward a determination of the exact 
number of either the chromomeres or their constituent parts, largely because 
the differential staining power of the two substances in the spirem is com- 
monly lost as the contraction of the thread proceeds, and the chromosomes 
appear homogeneous. An elaborate organization of the chromosomes, 
which are subdivided into ‘ chromomeres,’ and these into ‘ chromioles,’ 
has been described by Eisen (’00) for the spermatogenetic divisions of 
Batrachoseps. 
It was first observed by Pfitzner (’81) and Flemming (’81) that the 
longitudinal splitting of the spirem is preceded by a fission of the chromo- 
meres ; and the common assumption has been that this process involves 
also a fission of each of the smaller granules of which the chromomere 
is composed. This assumption is usually incapable of proof by direct 
observation ; but it is supported by Brauer’s (’93) discovery of a fission of 
each of the chromatin granules in the thread \ these granules are very 
small at this stage, and later combine into a much smaller number of 
chromomeres. 
Weismann (’92) identified the darkly-staining chromomeres with his 
hypothetical ‘ ids,’ or ‘ ancestral germ plasms,’ each of which contains within 
itself all of the qualities of the species, but differs from its fellows by 
the possession of a certain ancestral combination of individual characters. 
This notion of the identity of the c id ’ with one of the visible chromatin 
bodies was accepted by Strasburger (’94). He considered the ‘ id ’ as made 
up of a number of smaller chromatin granules, together with more or 
less linin. Brauer (’93) thought that the chromatin granules, rather than 
the chromosomes, are the elements which retain an individual existence 
from one mitosis to the next. From the facts that have been mentioned 
regarding the formation of the chromatin bodies in the spirem by an aggre- 
gation of smaller granules, it is plain that the use of such terms as ‘ id,’ 
‘ chromomere,’ or ‘ chromiole ’ cannot imply any certain homology between 
the structures to which the same name is applied in different species. As 
