A Comparative Study of tlie Chi'omosomes etc. 
259 
sperniatids, as will be shown fartlier on, they can certainly be associatecl 
with the so-caUed “sex-chromosomes”. I shall therefore designate them 
as the xy pair; x being darker, more homogeneous, and richer in chromatin 
than y, its mate. 
The beginning of the strepsistene condition is characterized by the 
Separation of the threadlike chromosonies forming each of the double 
loops. That this Separation coincides with the line of apposition of the 
chromosomes during syndesis cannot be doubted, for at no time has 
there been a complete fusion of the components of the double threads, 
the light longitudinal cleft being always evident. The Separation niay 
begin either at one end or along the middle of each pair (Plate XIX, 
Figs. 60, 61). Often both ends of a pair remain connected while the 
chromosomes lengthen out and twist about each other (PlateXIX,Figs. 62, 
64, 65), Later they stain more diffusely, thus becoming less distinct than 
in the early strepsistene stage. 
The components of the xy pair separate along their entire length 
early in the strepsistene stage (Plate XIX, Fig. 61), but still retain their 
deep stainability. The ^-component now shows its beaded conditions 
to a less extent than at an earlier stage, and stains nearly like the auto- 
somes, but darker. The aj-component shortens and remains deeply 
stained (Plate XIX, Figs. 62—65). 
The formation of the tetrads from the twisted strepsistene timeads 
will be described in more detail in E. curvata, where the process can be 
observed with greater clearness. In brief, it is this: the partially sepa- 
rated halves of the paired twisted threads shorten, stain more deeply, 
gradually untwist and each component of a pair splits longitudinally. 
The longitudinal split is a true division of each chromosome, such as 
occurs in the early prophase of the somatic and spermatogonial mitoses, 
and can in no way be confused with the Separation of the components 
of the diploid threads which were formed by the parallel union of pairs 
of chromosomes in syndesis. Often the pairs of chromosomes remain 
connected at both ends, forming rings. EspeciaUy is this evident in 
the macrochromosome pair. Soon, however, the two extremities where the 
chromosomes are joined together to form rings, become diffcrentiated 
from each other. That which is to persist in the formation of the tetrad 
corresponds to the short arms of the cross-shaped tetrads and marks the 
point of Separation of the chromosomes in the first spermatocyte division. 
Each chromosome is flattened or band like and this end is somewhat 
broader than that opposite. IVlien one views face wise the ends of the 
chromosomes that remain in contact during the formation of the tetrad. 
