A study of the Chromosomes in three species of Pseudococcus. 49 
tendency is again lost at times shortly before division (fig. 33 a), but in 
the other two species it is maintained throughout. The remaining five 
chromosomes which had their origin in the more diffuse chromatin can 
be traced similarly Step by step through their development to their position 
in the plate. Here they are arranged more or less irregularly around the 
central group of closely associated chromosomes, and show no definite 
spatial relation with each other (fig. 10, 33 b and 44). 
To sum up — : there are ten chromosomes composing the metaphase 
plate of the first spermatocyte division, and this in an animal in which 
the somatic number of chromosomes is ten. Moreover, these ten chromo- 
somes are not tetrads, a point which will be still further emphasized in 
the discussion of somatic, spermatogonial, oogonial and oocyte stages. 
Identical in their extemal appearance when fully Condensed, the chromo- 
somes differ only in their development and in their arrangement in the plate. 
The distinct and unmistakable split arises at a fairly early stage in the 
course of the development of the chromosomes and is not as far as can 
be determined, evidence of a Separation of conjugated leptotene threads. 
At a certain stage (fig. 5 and 8) the split chromosomes bear a marked 
similarity to the split X-chromosome of Lygaeus as figured by Wilson 
('12). Like this, each chromosome is composed of equal halves, and the 
equality seems to extend even to the granules or chromomeres ranged 
along each half. The nature of this split and its subsequent development 
force me to the conclusion stated by Wilson for the X-chromosome 
split, that it is an indication of an approaching single division — not of 
tetrad structure. Certainly we are not dealing here with a Separation 
line between whole chromosomes. 
The ensuing division witnesses the distribution of ten chromosomes 
to each of the two daughter cells. Änd as already pointed out, this divi- 
sion takes place along the split. It is plain that it corresponds to our 
conception of an equation division. Although an essentially normal 
division, one or two features are sufficiently interesting to be emphasized 
here. Düring the later anaphases, while the chromosomes are still moving 
toward the poles, each daughter set can be seen to be arranged in two 
groups — one slightly preceding the other. Since good evidence of this 
can only be obtained from side views, chromosome countS are more or 
less difficult. However, very good figures are occasionally obtained, and 
these show plainly that the ten chromosomes approaching a pole are 
divided into two groups of five each (fig. 34). Furthermore when the 
pole has been reached, the chromosomes are once more arranged in a 
plate, and this arrangement corresponds to that observed in the parent 
Archiv f. Zellforschung. XVII. 4 
