576 SCIENCE PROGRESS 



one spindle) in end view, all the chromosomes being in 

 evidence, six of them at one pole and seven at the other. 



(Cf. fig. 2.) 



In a second type the X-chromosome is double, consisting of 

 two components which pass together to one pole. Examples of 

 this are Syromastes (Gross), Phylloxera (Morgan), and Agalena 

 (Wallace). The X-spermatozoa receive in these cases two more 

 chromosomes than the Y-class (fig. 2). 



In a third and very common type, widely distributed among 

 the insects, the X-chromosome is accompanied by a synaptic 

 " mate " or " partner " (usually smaller than X) which passes 

 into the Y-class of spermatozoa, and may therefore be called 

 the Y-chromosome or " Y-element." 1 In one of the maturation- 

 divisions (the first in Coleoptera and Diptera, the second in 

 Hemiptera) X and Y are found in close contact, but as the 

 division proceeds they separate and pass to opposite poles, 

 while all the other chromosomes divide equally. The final 

 result is that all the spermatozoa possess the same number of 

 chromosomes, but half contain X and half Y (Lygceus, fig. 2 ; 

 Euschistus, fig. 1, /). Here again it is certain in some species, 

 and extremely probable in all, that the X- and Y-chromosomes 

 are of constant identity. Comparative studies have made it 

 very probable that from this type, by the disappearance of 

 the Y-chromosome, has arisen the first type (with an odd or 

 " accessory" chromosome); for various intermediate stages have 

 been found in different species between those in which Y is 

 almost as large as X (Benacus, Mineus) and those in which it 

 is very minute (as in many Coleoptera and Diptera). Its final 

 disappearance would leave the X-chromosome without a mate. 

 On the other hand, there are some forms (e.g., Nezara, Onco- 

 peltus, fig. 2) in which X and Y are equal in size, and cannot 

 be distinguished by the eye ; but these two chromosomes agree 

 precisely in all other respects with the X- and Y-chromosomes 

 of other species. The possibility is thus suggested that there 

 may be many cases in which the X- and Y-chromosomes may 

 be present, though not visibly different from the others. 2 



From the types just described, with both X- and Y-chromo- 

 somes, an interesting series may be traced in another direction, 



1 In this type X and Y are identical respectively with the chromosomes which 

 have been called the large and the small " idiochromosomes." 



2 See Wilson, Journ. Exp, Zool. 1905-6. 



