392 INTRODUCTION TO CYTOLOGY 



accepted by some geneticists as the desired material genes; but, as pointed 

 out in the chapters dealing with somatic mitosis and reduction, many 

 cytologists are very uncertain as to the morphological status and signifi- 

 cance of these bodies, which seem to them to be far too inconstant in 

 number and behavior to represent the units in question. Although it is 

 tempting to look upon the chromatic granules as the units which current 

 theories of heredity seem to require, it must be admitted that the 

 observational evidence is insufficient to warrant the categorical state- 

 ments frequently made to the effect that the chromosome is composed of 

 a definite number of more elementary visible chromatic units, which 

 have definite space relations and are the significant units in the cellular 

 mechanism of heredity. On the other hand, the careful observations of 

 Wenrieh (1916) have shown that in the grasshopper, Phrynotettix (Fig. 

 155), the chromatic granules are relatively constant in size and position 



c * e f g h i & -^ 



fc 



Fig. 155. — Chromosome pair "B" in conjugation from the spermatocytes of 13 differ- 

 ent individuals of Phrynotettix magnus, showing constancy in size and arrangement of the 

 principal chromomeres. The same constancy is shown in the different cells of a single 

 individual. X 1500. {After Wenrieh, 1916.) 



in a given member of the chromosome complement, even in different 

 individuals; and they furthermore show a close correspondence in the two 

 homologous chromosomes as they pair at synapsis. This is one of the 

 most striking pieces of direct cytological evidence yet brought forward in 

 support of the theory that the chromosome is a "chain of factorial beads ' ; 

 (Harper), and heightens the probability that the postulated units of 

 inheritance will turn out to be more than purely conceptual ones. 



Whatever may be the value of the chromatic granules, one can hardly 

 fail to recognize the highly suggestive nature of the arrangement of the 

 chromatin in a thin thread, its frequently beaded appearance, and its 

 accurate longitudinal fission into two equal parts at the time of cell- 

 division. In the absence of direct and convincing cytological evidence 

 for the presence of various "qualities' 3 arranged in a series along the 

 thread, we may still look hopefully for the support which it would seem 

 that the theory of Roux must sooner or later have. It must be admitted 

 that at present the evidence for the existence of genes is in the main 

 genetical rather than cytological. 



