A STUDY OF SOMATIC CHROMOSOMES. 339 



view that the normal somatic chromosome number is a constant 

 one in Anasa. 



It is evident, then, that the embryos of Anasa tristis fall into 

 two classes with respect to their chromosome content, one con- 

 taining 21, and the other 22 chromosomes in all the cells of the 

 various tissues where an examination was possible, with the one 

 exception noted. These conditions are found in every plate 



" " 



f f I ( I I I I . 



9 



! I f i > i 



I c I i i i i i i 



10 



Anasa tristis. 22 chromosomes, female type. Fig. 9. Oogonium.' Fig. 10. 



Epithelium of oviduct. 



where it is at all possible to make an accurate count. In some 

 cases the m-chromosomes lie so near the largest chromosomes 

 that it is difficult to distinguish them. Even in metaphase 

 groups where a count is impossible, it is possible to identify 

 many of the chromosomes. 



Aside from the general constancy in the number of the chromo- 

 somes in these embryos, the most important and significant 

 fact is that of the constancy in type. The individuality of the 

 chromosomes is represented by this constancy of type, and it is 

 clear that in Anasa this is not confined merely to the chromo- 

 somes in the sex cells, but occurs throughout early embryonic 

 development in the somatic tissues, and it is logical to suppose 

 from this that this constancy in type of the chromosomes is 

 present throughout the entire life cycle of this animal. Of similar 

 importance is the fact that definite pairs of chromosomes can be 



