488 E. ELEANOR CAROTHERS 
known till they are proved different.’ Since the above was 
written the increased knowledge of Mendelian phenomena has 
shown that they occur in practically every group of organisms. 
I hope that it may be possible to determine what the characters 
controlled by this particular pair of chromosomes are, through an 
experimental study of the living animals, correlated with a study 
of the developing soma of the embryo. 
The credit for recognizing the importance of this tetrad belongs 
to Dr. C. E. McClung, as I was not sufficiently familiar with 
the ground to realize its general bearing when I found it. I am 
also indebted to him for consistent encouragement and guidance 
throughout the whole course of the work. The slides used were 
Dr. W. S. Sutton’s, a few of Dr. McClung’s and a number pre- 
pared by myself, from material in Dr. McClung’s collection. In 
the work on the so-called nucleoli, or plasmasomes, I have been 
enabled to compare a number of different genera through the co- 
operation of D.H. Wenrich, a fellow student, who is giving especial 
attention to the growth period. 
This work is based chiefly on Brachystola magna, a form already 
well known, so far as the general organization of the chromosomes 
is concerned, through the valuable researches of Sutton (’00, 702, 
03). On this account, and in view of the fact that another paper 
by the same worker is soon to appear dealing with Brachystola, 
I shall restrict this paper strictly to the subject in hand, with the 
understanding that upon all other essential points my observa- 
tions agree closely with those of Sutton. 
While this chromosome does not come under MeClung’s origi- 
nal definition of a multiple, since it is not united with any of the 
others during the metaphase, it frequently forms a hexad during 
the prophase by uniting with one end of the accessory. It further 
separates itself from the ordinary chromosomes by having the 
spindle fibers attached to the free, instead of the synaptical ends, 
and in consequence, dividing transversely in the first spermato- 
cyte when the other tetrads divide longitudinally. 
