CHROMOSOME RELATIONS IN ONISCUS. 29 



it seems safe to conclude that any correlation existing between 

 the shape of the chromosomes and the tangible bodily character- 

 istics is not very close nor exact. Species quite distinct from each 

 other may possess chromosomes of almost the same shape and size. 

 Especially is this true of -plant species. On the other hand, 

 species closely resembling each other may have chromosomes 

 of perceptibly different shape or size. This is even more obvi- 

 ously the case with the number of chromosomes. In some 

 instances a difference in number may be accompanied by differ- 

 ence in sex, 1 in others by specific differences. 2 Again the same 

 number may characterize groups as widely different as angio- 

 sperms, mammals and insects 3 ; while related species of the same 

 genus may differ in number. 



Nevertheless in a general way there appears to be some cor- 

 respondence between the bodily attributes of a group and the 

 character and behavior of the chromosomes of that group. 

 Chromosomes of a particular shape are more characteristic of 

 certain groups than of others. Thus, the form in which the 

 limbs of the bivalent chromosomes are twined about each other 

 is frequently found in flowering plants and also in amphibia, the 

 ring form is particularly apt to occur in arthropoda and a shape 

 somewhat like a shepherd's crook, with a slender body and thick- 

 ened, curved extremity, is often seen in mollusks and worms. 

 Again the peculiar chromosome known as the accessory so far 

 has not been found outside of the Tracheata. 



No doubt the shape, possibly also the number of chromo- 

 somes, is determined by complex physical and chemical condi- 

 tions which may not have any direct relation to visible bodily 

 characters and two chromosomes of the same shape and size 

 may contain potentialities of quite different nature, just as an 

 apple and its waxen imitation, although of the same size and 

 shape, have very diverse properties. 



A parallel condition may be seen in the relation of the shape 

 and size of the egg to the bodily characters of the organism 

 developing from it. Forms differing as widely as fish and 



1 E. B. Wilson, "The Sexual Differences of the Chromosome Groups in 

 Hemiptera," Jour. Exp. Zoo!., III., No. I, 1906. 

 2 Montgomery, Morgan, Stevens, loc, cit, 

 3 E. B. Wilson, "The Cell in Development and Inheritance," p. 206, igoo. 



