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cells. Della Valle, in a paper in wliicli he himself describes 
half-a-dozen nuclei with tetrads from various tissues of the 
salamander, and from Bidder’s organ in the toad, has 
collected together a large number of references; Gregoire 
has added to the list, and from these two authors it may be 
seen how widely distributed such occurrences are throughout 
both the animal and vegetable kingdoms. It is interesting 
to note that Gregoire and Wygaerts figure three transversely 
segmented metaphase chromosomes of Trillium, in all of 
which they are divided by the joint into two unequal lengths, 
and that on p. 15 they state that the commonest form of 
anaphase chromosome in this plant “ est cede d’un V incom- 
plet, forme d’nne grande et d’une petite branche.” 
Besides della Valle, very few authors have published 
researches directed specially to the occuri-euce of transverse 
divisions in somatic chromosomes. Popoff has described 
the appearance of tetrads in the liver-cells of Pain din a, 
generally in the full somatic number. These are closely 
similar to the tetrads appearing in the primary oocytes of the 
same animal, and Popoff (like della Valle) considers that in 
both cases they are due to a physiological abnormality of 
the cells in question. 
An inquiiy into somatic tetrads has been made by Hacker 
and his pupils. Hacker brought out the transverse segmen- 
tation of the chromosomes in developing Copepod eggs with 
diagrammatic distinctness by the action of ether. This has 
been done still more beautifully by Schiller, working on 
several species of Cyclops. His figures 7-15 show typical 
tetrads, present of course (except in rare cases) in the somatic 
number. 
Nemec has brought about the formation of tetrads in 
somatic plant tissues by the action of chloral hydrate. 
This brief reference to the literature will be enough to 
show that the tendency to transverse segmentation of chromo- 
somes is very widely distributed throughout the animal and 
vegetable kingdoms — probably, indeed, the potentiality to 
such segmentation is present in all chromosomes, and becomes 
