McAllister—Cytology and Embryology. 603 
the hybrid as allelomorphs. Since in the germ cells of these 
varieties there are only seven chromosomes it would be necessary 
that the 18 allelomorphs must occur in seven groups, which 
Farmer regards as improbable if not impossible. He regards 
the chromosomes as made up of groups or rows of chromomeres, 
—“but they would not necessarily represent permanent and per¬ 
sistent structures in the sense that each one is to be looked at as 
being invariably composed of the same chromomeres.” 
The possibility of the pairing of paternal and maternal 
chromosomes to form the heterotypic chromosomes seems to 
have been first urged by Henking (40) in his attempt to ex¬ 
plain the origin of the tetrad chromosomes of Pyrrhoeoris. 
Montgomery (58) figured a series of stages and described the 
pairing of the chromosomes in the spermatogenesis of Peripatus 
balfouri. In 190,1 he argued that in Asearis megalocephala 
univalens pairing of paternal and maternal chromosomes to 
form the heterotypic chromosome necessarily follows since in 
the diploid nuclei there is but one chromosome from each parent. 
He (60) laid the foundation for the study of the sex chromo¬ 
some by advancing further evidence based upon studies of 
Hemiptera to the effect that pairing of parental chromosomes 
here takes place. In Hemiptera heteroptera two small “hetero¬ 
chromosomes” are present in the division stages of the diploid 
nuclei. These unite during synapsis to form one double heter¬ 
otypic chromosome and are separated at the time of the heter- 
typic metaphase so that each germ cell contains but one “het¬ 
erochromosome.” Evidently then in the fertilized egg one 
“heterochromosome” must come from the male parent and one 
from the female and it is these two which pair later. From 
these observations Montgomery argues that the other double 
chromosomes of the first division must be formed in the same 
manner. 
Winiwarter (108) in 1901 described paired threads as oc¬ 
curring during synapsis in the nuclei of the mother cells of 
the rabbit and man. This paired condition he connected with 
the origin of the heterotypic chromosomes, reasoning that the 
