26 
THE ORGANIC CELL 
arranges itself into masses, each mass divides into a group of 
four bodies which are connected by linin threads, forming 
what is known as a tetrad. The number of tetrads is always 
one half the usual number of chromosomes. In Ascaris two 
tetrads appear in the germinal vesicle, the normal number 
Formation of Polar Bodies in Ascaris Megalocephala 
var. Bivalens. (Boveri.) 
A. The egg, with sperm entering at X ; the germinal vesicle contains two 
tetrads, the number of chromosomes in the species being four. B. First polar 
spindle. C. The tetrads dividing. D. First polar body formed, containing 
like the egg, two dyads. E, F. The dyads rotating into position for the second 
division, p.b. first polar body. G. The dyads dividing. H. Each dyad has 
divided into two single chromosomes, thus completing the reduction, the two 
at the periphery forming the second polar body p.b 2 . 
of chromosomes being four. During the formation of the 
first polar body, each tetrad becomes halved, to form two 
double groups or dyads ; one group of dyads remains in the 
egg, the other joins the polar body. It is therefore evident 
that both the polar body and the egg receive a number of 
