HETEROCHROMOSOMES IN MOSQUITOES. 113 



The character of the chromosome pairs as they appear in the first 

 maturation mitosis is well shown in Figs. 14-17, where the chro- 

 mosomes are represented in one plane for the sake of clearness. 

 In each figure x is the composite pair. The spindle fibers appear 

 to be attached at the meeting point of the smaller and larger 

 components, the smaller, unequal pair forming the outer ends 

 of the figure and appearing as a conspicuous hook at one end and 

 a rounded knob at the other. No distinct separation of the 

 members of the composite pair, such as is always seen in the 

 spermatogonia (Figs. 3 and 4) was ever observed in the spermat- 

 ocyte metaphase. The other two chromosomes frequently 

 assume the form of rings in the early metaphase (Figs. 16 and 17) 

 and in the anaphase (Fig. 18) they separate as V's with equal 

 arms, while the heterochromosomes form a pair of hooks with 

 equal long arms and unequal short arms (Fig. 18). 



Fig. 19 shows the chromosomes of a first spermatocyte meta- 

 phase of Culex pipiens and Fig. 20 an anaphase. The chromo- 

 somes correspond very closely in form and relative size to those 

 of Anopheles with the exception that all three pairs are equal. 

 Both cells and chromosomes are much larger in Culex than in 

 Anopheles. 



In the second spermatocytes the chromosomes are already 

 divided when they come into the spindle and are usually so 

 tangled as to defy any attempt to distinguish the two classes or 

 make clear drawings. In Fig. 21 we have an unusually clear 

 metaphase. An aggregation of chromatin, apparently the hetero- 

 chromosome, is found in the spermatids (Fig. 22), lying against 

 the nuclear membrane, and in some cases the dimorphism can be 

 distinguished. 



Theobaldia incident. 



Theobaldia incidens is one of the most common mosquitoes in 

 the Santa Clara Valley, California. The larvae were found in 

 abundance in a bucket of water which had been allowed to stand 

 partly covered for two or three weeks. A few bread crumbs 

 thrown into the glass jars in which the larvae were kept under 

 observation, enabled them to grow rapidly and transform into 

 fine large pupae whose germ glands yielded all desired stages, 

 from dividing oogonia and spermatogonia to ripe spermatozoa. 



