104 PAULINE H. DEDERER. 



form, very granular in appearance, and stain less deeply than the 

 spireme. 



Concentration of Rings. 



This is the next well-marked stage. The threads forming the 

 rings become thicker, rougher in outline, and more deeply stain- 

 ing (Fig. 26), and as they thicken their circumference decreases 

 until the originally large central space is reduced to a minute 

 cavity, which is finally closed up altogether, and a chromosome 

 results. Fig. 27 shows various stages in concentration. Chro- 

 mosomes as shown in Fig. 28 and 29 succeed this until the at 

 first irregular elliptical masses have assumed the smooth dumb- 

 bell-like form seen in the first prophase stage (Fig. 30). 



During concentration of the rings, the idiochromosome also 

 appears to thicken, becoming shorter and broader, so that it has 

 a smaller surface of contact with the plasmosome (Fig. 19,/, and 

 28). In Fig. 19, g, from anearlyprophase.it has the smooth 

 bipartite form common to the other chromosomes. From this 

 time, the plasmosome disappears, and the idiochromosome is 

 indistinguishable from the others. 



Conclusion. 



The presence of a double idiochromosome in Philosamia con- 

 nects it in this respect, with Envanessa and Caccecia, two species 

 of butterflies studied by Miss Stevens ('06), who finds an equal 

 pair of idiochromosomes, or "sometimes a tzvo lobed body," . . . 

 " whose only apparent peculiarity is its condensed form during 

 growth." 



In the case of the moth the idiochromosome appears single 

 from the time of its first appearance, but it would seem that it is 

 a bivalent body, in just the same way that the rings and resulting 

 chromosomes are bivalent. This bivalence has its origin in all 

 probability, in the prespireme stage. 



Philosamia thus lies at the opposite end of a series, from 

 Nesara, where the equal idiochromosomes do not unite until 

 after the first division. An intermediate stage is represented in 

 Brocliymena (Wilson, '05), where the idiochromosomes, in this 

 case unequal, lie at first separated, but later united, in the growth 

 period. Wilson concludes for this form that, " when only one 



