418 EDMUND B. WILSON 



Again, from the existence of tetrad-shaped chromosomes in cer- 

 tain somatic divisions of Amphibia, Delia Valle ('07) concludes, 

 "Tutte le precedente formazioni emolto probabilmente, quando 

 esistono, anche quelle della profase del primo fuso di matura- 

 zione, non hanno alcun rapporto con la reduzione cromatica, ma 

 sono indice di una costituzione patologica dei cromosomi"(!) 

 Both the foregoing passages, I think, like others of like import that 

 might be cited, overshoot the mark. The significance of ring- 

 shaped or tetrad-like chromosomes must be judged from a more 

 critical standpoint than this. We must endeavor to discover their 

 real meaning in each case by the study of their origin, their behavior 

 in successive divisions, their morphological composition (whether 

 simple or compound bodies), their relation to the conditions 

 seen in other species, and any other facts that may throw light 

 upon them. By way of illustration certain specific cases may be 

 mentioned. A very interesting one is afforded by the X-chromo- 

 some of Syromastes (Gross, '04, Wilson, '09 a, '09 b), which is unac- 

 companied by a synaptic mate (F-chromosome) yet forms a con- 

 spicuous 'tetrad' in the first spermatocyte-di vision. The reason 

 here is that this ' chromosome' consists of two components, 

 which appear as separate chromosomes in the spermatogonial 

 groups but in the maturation-divisions are associated to form a 

 double element which behaves precisely like the single X-chromo- 

 some of other species, and obviously corresponds to the latter 

 because four such components are present in the diploid groups 

 of the female (Wilson, '09 b). The '^chromosome' of Nezara 

 is a different but not less interesting case, always forming a con- 

 spicuous 'tetrad' in the second spermatocyte-division, but appear- 

 ing as a single chromosome in the diploid groups. These two 

 apparently contradictory cases are brought under the same point 

 of view especially when compared with the analogous relations 

 described by Morgan ('09) in Phylloxera, and by Browne ('10) 

 in Notonecta if we assume that in each case the chromosome in 

 question was originally a single body which in Nezara shows a 

 slight tendency to separate into two components, in Syromastes 

 has already done so (cf. Wilson, '11 a). Neither case offers 



