332 W. E. HOY, JR. 



All the figures were drawn r at table level with the aid of an 

 Abbe camera lucida, using a Leitz 18 compensating ocular and 

 a 2 mm. apochromatic objective, and are reproduced here at a 

 reduction of one third. In a few cases it was necessary for the 

 sake of clearness in the reproductions to draw one or two chromo- 

 somes out of their true positions. The chromosomes linearly 

 arranged were drawn from the actual specimens, and not traced 

 from the drawings of the plates. 



Mitotic figures occur throughout the embryos and are numer- 

 ous, but it must not be supposed that the chromosomes in many 

 of these figures can be counted. It is sometimes necessary to 

 section twenty-five or more embryos before finding a division 

 figure which is clear. In this connection the remarks of Metz 

 ('16, p. 215) are pertinent: "Although certain principles must 

 be observed in making preparations, the task is mainly one of 

 securing and preparing enough specimens to get material in the 

 proper stages and in sufficient quantity for study." 



OBSERVATIONS. 



Anasa tristis is an exceedingly favorable form for this par- 

 ticular problem, since the chromosome complex is so well known, 

 and since it has been described by several investigators with 

 general unanimity in result. The striking size differences of the 

 chromosomes of one plate also makes it an excellent form for 

 study. 



In order to understand the nature and character of the chromo- 

 some complex, it will be necessary to review briefly the description 

 of the spermatogenesis and oogenesis of this species. Wilson 

 ('055) ('06) showed that the number of chromosomes appearing 

 in the spermatogonial divisions was 2 1 . Three of these chromo- 

 somes are larger, and two are very much smaller, than the 

 others, so that there is a marked differentiation in size in any one 

 group. Of these 21 chromosomes, 20 can be paired, leaving one 

 the largest, unpaired. This largest chromosome is the ^-chromo- 

 some (accessory). The two smallest have been termed the m- 

 chromosomes, and are distinguished by the fact that they do 

 not unite to form a bivalent chromosome in the growth period, 

 but condense as two separate chromosomes, pairing in the first 



