410 EDMUND B. WILSON 



fusion of the two conjugants;and that the synaptic process involves 

 not merely an association of the chromosomes to form 'gemini' 

 but a process of reconstruction which may profoundly change 

 their composition (cf. Boveri, '04). I am, however, by no means 

 in agreement with those writers who for a similar reason would 

 reject in toto the conception of the reduction-division in the case 

 of these chromosomes. Very important evidence upon this point 

 is afforded by the contrast in structure and behavior between bival- 

 ents and univalents in the maturation-divisions ; and this has not yet 

 received sufficient attention on the part of writers on this general 

 subject. In the first place, it is a rule, without exception so far as I 

 am aware, that univalent chromosomes divide but once (of course 

 equationally) in the course of the two maturation-divisions, while 

 bivalents divide twice. The additional division in case of the 

 bivalents must, therefore, be in some manner a consequence of 

 synapsis. In the second place, the difference between univalents 

 and bivalents is often clearly displayed in a characteristic differ- 

 ence of structure in the prophases. In the insects that I 

 have studied the former are always bipartite bodies, the latter 

 often quadripartite obviously in preparation for a single divi- 

 sion in the former case, for two divisions in the latter. The best 

 examples of these facts are offered by the sex-chromosomes; but 

 they are also exhibited by the w-chromosomes of Hemiptera, and 

 by certain anomalies sometimes seen in the autosomes. 



Perhaps the most striking of these cases is that of the X-chromo- 

 some because of the different conditions seen in different species. 

 In some forms this chromosome is accompanied by a synaptic 

 mate (the F-chromosome) with which it unites to form a bivalent 

 before the spermatocyte-divisions (Coleoptera, Diptera) ; in other 

 species X and Y divide as separate univalents in the first division 

 and afterwards conjugate (many Hemiptera) ; while in still others 

 Y is missing and X is always univalent. In the first case the XY- 

 bivalent 'divides' in both spermatocyte-divisions reductionally 

 in the first, equationally in the second, as may be clearly seen 

 because of the inequality of X and Y (Stevens). In the second 

 case (e.g., Oncopeltus Lygaeus) this order is reversed, the first 

 division being of course equational, the second reductional. In 



