CHROMOSOMES OF MOSQUITO 533 



homologues. If these splits be continued until they intercept 

 each other, there will result a condition like text figure 4, in 

 which one chromatid from each chromosome appears to cross 

 over and unite with the other chromatid from the other chromo- 

 some. If we suppose the primary and the secondary splits to 

 be widened so that the ends of the chromosomes are pushed 

 apart until they all lie in the same plane, there will be formed a 

 cross like text figure 5. The horizontal arms of the cross are 

 homologues. The vertical arms are fused daughters of homo- 

 logues. The spindle-fibers may be supposed to attach to the 

 middles of the chromatids. If the chromosomes of text figure 

 5 separate in a horizontal direction the division will be reduc- 

 tional. If they separate vertically the division will be equa- 

 tional. Text figure 6 represents an equational division. Text 

 figure 7 represents a simple crossing of two equationally divided 

 chromosomes. 



It now becomes evident that it is impossible to determine in 

 Culex pipiens whether the firs f t division is reductional or equa- 

 tional. In the first place the members of the pairs as they appear 

 in late prophase of the first spermatocyte may represent homo- 

 logues or they may represent fused daughters of homologues. 



Figure 48 shows the three pairs at a somewhat later stage 

 than figure 39. Where the members touch each other there 

 may be a simple crossing like text figure 7, or a separation in 

 different planes like text figure 4. 



In figure 39 the ends of the larger pairs are united. It is 

 impossible to tell whether the split which separates the middles 

 is primary or secondary. In later stages appearances suggest 

 very strongly alternations of splits at various points of the 

 chromosomes. Where this occurs the chromatids may be said 

 to be 'changing partners.' In figure 49, from another cell of 

 the same cyst as figure 48, points a, b, and c are clearly simple 

 crossings of the fibers; point d suggests very strongly a 'change 

 of partners.' 



Figure 50 is a later stage in which each of the three pairs 

 shows a change of partners at one end. In pair a this has pro- 

 gressed well toward the middle. In this and the succeeding 



