SYNAPSIS AND CHROMOSOME ORGANIZATION 507 



Chorthippus are of the compound type and that those of Tri- 

 merotropis belong to the other type. Considering the similarity 

 of organization of these chromosomes in the two species, i.e., the 

 presence of the apical constriction in the two types, it will be 

 seen that such a constriction cannot be used as a criterion for the 

 recognition of the compound type. 



Robertson seeks to extend this compounding idea to the 

 V-shaped chromosomes of other groups of animals. It seems to 

 be unsafe to generalize so extensively, considering that such a 

 generalization does not seem to hold in all cases, even among 

 the Acrididae. It would appear, rather, that each group would 

 have to be considered by itself and conclusions drawn only from 

 a study of numerous species within a group. Such a study has 

 been made by Metz ('14) in which case the compound nature of 

 the V-shaped chromosomes of Drosophila seems to be established. 

 But in this case, as Metz has pointed out, it is as admissible to 

 consider two rods to have arisen from the transverse cleavage of 

 a primitively V-shaped chromosome as to assume that a V-shaped 

 type arose by an end-to-end union of two of the primitively rod- 

 shaped type. As Robertson has noted, Agar ('12) found very 

 decidedly constricted chromosomes in Lepidosiren and that (p. 

 295): ''The constancy of the position at which transverse seg- 

 mentation takes place indicates a constant differentiation of the 

 chromosomes in a lengthwise direction." But he considered that : 

 ''The presence of transverse constrictions or joints in chi'omo- 

 somes is not, without special evidence, to be taken as an indica- 

 tion of bivalency or of a future division plane." This note of 

 caution ought, I think, to be especially emphasized. 



b. The chromomere-vesicles. The plasmosome-like chromo- 

 mere-vesicles were first called 'vesicles' by Carothers ('13). In 

 '16 I called attention to their plasmosome-like appearance, at 

 least in certain stages, and emphasized the point, already noted 

 by Carothers, that they are related to definite regions of chromo- 

 somes. Dr. Carothers ('17) now calls them ' chromomere 

 vesicles' (p. 473), a term which emphasizes their relationships to 

 separate chromomeres and distinguishes them from the chromo- 

 some vesicles found in the spermatogonia where each vesicle 



