SEGMENTATION AND DIEFERENTIATION OF CHRO.MOSOMES. 293 
operative especially often when they become^ for any reason, 
unusually short in proportion to their length. In this case 
the transverse constriction together with the metaphase split 
gives the well-known tetrad appearance. 
The only constructive theory as to the significance of this 
tendency to transverse segmentation that is known to me is 
Hiieker’s theory of teleutosyndesis. It is unnecessary to 
dwell long on this, however, as it seems very improbable that 
it can be maintained in the face of the facts brought forward 
in this paper. For, firstly, the theory requires the final 
fusion of the end-to-end segments at some stage in the life- 
cycle to prevent the number of segments in each chromosome 
being doubled in each meiosis. It is extremely improbable 
that such a fusion ever takes place in Lepidosiren, as the 
“doubleness” is traceable in all the tissues of the animal, 
and is specially well marked in the daughter-plates of the 
spermatogouial divisions — tliat is, in the last divisions before 
the meiotic prophase where the end-to-end conjugation is 
supposed to take place again. Secondly, the constant 
inecpndity of the two segments of many chromosomes is very 
much against the theory that they have been formed by the 
permanent pairing end-to-end of pairs of homologous ones. 
Thirdly, the theory is in any case of such a nature as could 
only be accepted on very strong direct evidence in its 
favour. 
GeNEKAL CoNSIDEKATlONS AS TO THE NaTUKE AND SIGNIFI- 
CANCE OF THE 'TrANSVEKSE SEGMENTATION. 
The immediate cause of the transverse segmentation may 
well be physical — j)erhaps the tendency may be for the 
chromatin to collect at the ends of the chromosomes and fiow 
away from the middle. This is possibly dependent on purely 
])hysical factors — perhaps electrical charges or surface ten- 
sion, both of which have often been supposed to play a large 
part in the movements observable in mitosis. In accordance 
with this view is the fact that the constrictions are best 
