i66 



Edmund B. Wilson 



and that, accordingly, the usual rule holds in regard to fertiliza- 

 tion. 



Exceptional conditions. There are two conditions, rarely seen, 

 that are of interest for comparisons with other species. Now and 

 then the idiochromosomes fail to separate for the first division, 

 but remain in more or less close union to form an asymmetrical 

 bivalent, which in side view is seen to form a tetrad (Figs. 4, f-i, 

 ky Photo 3). This bivalent undergoes an equation division, in 

 this respect agreeing with the conditions uniformly seen in Syro- 



FiG. 6 

 M. femoratus (No. 29) 22-chromosome form 

 Four nuclei from growth-period showing diffused ordinary chromosomes, condensed cfiromosome- 

 nucleoli and plasmosome; in a and h the two idiochromosomes are united to form double chromosome- 

 nucleoli (Photo 25); in c and d they are separate (Photo 26). 



mastes (Gross '04, Wilson '09), and differing from that occurring in 

 the Coleoptera or Diptera (Stevens '06, '08a). A rarer but more 

 interesting deviation from the type is the failure of the idiochro- 

 mosomes to separate in the second division, both passing together 

 to the same pole (Fig. 5, o, p, Photo 18). Since the other chromo- 

 somes divide equally it may be inferred that in this case one pole 

 receives 12 chromosomes and the other but 10. This has been 

 seen in only three cells and is doubtless an abnormality. It may 

 however, possess a high significance as forming a possible point 



