STUDIES ON CHROMOSOMES 387 



ing 'Study 7 ('11 a) that the X-chromosome (like other chromo- 

 somes) is a compound body. They help us to understand how an 

 X-chromosome that is originally single may break up into two 

 or more components that behave as separate chromosomes in the 

 diploid groups but become associated in a coherent group ('X- 

 element') at the maturation-period (Payne, '09, Wilson, '11, 

 Edwards, '10), and provide a still more definite basis for the con- 

 clusion that this chromosome may be the bearer of many other 

 factors than the one for sex (Wilson, '11, Morgan, '11, Gulick, '11). 

 The bearing of this on sex-limited heredity is obvious. 



It is a very important fact that at no time in their history do the 

 individual sex-chromosomes in these Hemiptera exhibit a cross- 

 form or tetrad structure comparable to that which is so characteristic 

 of the bivalents. Such a tetrad structure only appears when the 

 two sex-chromosomes are united to form a bivalent as is seen 

 for instance in Brochymena or Nezara (Wilson, '05 b, '11 a). 

 The only apparent exception to this is the .XT-chromosome in 

 Lygaeus, as already mentioned; but this exception is evidently 

 only apparent. The essentially bipartite structure of these chro- 

 mosomes is a significant fact that is obviously correlated with 

 their univalent nature, and with their approaching single divi- 

 sion in the course of the two spermatocyte-divisions. The wider 

 implications of this will be considered in Part III, in connection 

 with the facts seen in Protenor. 



6. Comparative considerations regarding the maturation-period 



A comparison of the growth-period in these Hemiptera with the 

 conditions seen in such forms as Tomopteris or Batracoseps shows 

 some striking, though I think secondary points of difference. 



In the first place, the formation of compact, massive bodies 

 from which the leptotene-threads unravel in the pre-synaptic 

 period, which is so characteristic of these insects, seems not to 

 take place in Batracoseps and some other forms; though, as will 

 be indicated beyond, the Schreiners have found indications of an 

 analogous process in Tomopteris. 



Secondly, the polarized amphitene, or 'bouquet-stage,' that is 

 characteristic of Tomopteris, Batracoseps and other forms, seems 



